#the short summary of why i want to find a web-based epub reader is:

[moonlit-tulip]

What's your favorite ebook-compatible reading software? Firefox EPUBReader isn't great, but I'm not what, if anything, works better.

Very short answer: for EPUBs, on Windows I use and recommend the Calibre reader, and on iOS I use Marvin but it's dying and no longer downloadable so my fallback recommendation is the native Apple Books app; for PDFs, on Windows I use Sumatra, and on iOS I use GoodReader; for CBZs, I use CDisplayEx on Windows and YACReader on iOS; and I don't use other platforms very often, so I can't speak as authoritatively about those, although Calibre's reader is cross-platform for Windows/Mac/Linux, and YACReader for Windows/Mac/Linux/iOS/Android, so they can serve as at least a minimum baseline of quality against which alternatives can be compared for those platforms.

Longer answer:

First off, I will say: yeah, Firefox EPUBReader isn't great. Neither, really, are most ebook readers. I have yet to find a single one that I'm fully satisfied with. I have an in-progress project to make one that I'm fully satisfied with, but it's been slow, probably isn't going to hit 1.0.0 release before next year at current rates, and isn't going to be actually definitively the best reader on the market for probably months or years post-release even assuming I succeed in my plans to keep up its development. So, for now, selection-of-ebook-readers tends to be very much a matter of choosing the best among a variety of imperfect options.

Formats-wise, there are a lot of ebook formats, but I'm going to collapse my answers down to focusing on just three, for simplicity. Namely: EPUB, PDF, and CBZ.

EPUB is the best representative of the general "reflowable-text ebook designed to display well on a wide variety of screens" genre. Other formats of similar nature exist—Kindle's MOBI and AZW3 formats, for instance (the latter of which is, in essence, just an EPUB in a proprietary Amazon wrapper)—but conversion between formats-in-this-broad-genre is generally pretty easy and not excessively lossy, so you're generally safe to convert to EPUB as needed if you've got different formats-in-this-genre and a reader that doesn't support those formats directly. (And it's rare for a program made by anyone other than Amazon to work for non-EPUB formats-in-this-genre and not for EPUBs.)

PDF is a pretty unique / distinctive format without any widely-used alternatives I'm aware of, unless you count AZW4 (which is a PDF in a proprietary Amazon wrapper). It's the best format I'm aware of for representations of books with rigid non-reflowable text-formatting, as with e.g. TTRPG rulebooks which do complicated things with their art-inserts and sidebars.

And CBZ serves here as a stand-in for the general category of "bunch of images in an archive file of some sort, ordered by filename", which is a common format for comics. CBZ is zip-based, CBR is RAR-based, CB7 is 7-zip-based, et cetera; but they're easy to convert between one another just by extracting one and then re-archiving it in one's preferred format, and CBZ is the most commonly distributed and the most commonly supported by readers, so it's the one I'm going to focus on.

With those prefaces out of the way, here are my comprehensive answers by (platform, format) pair:

Browser, EPUB

I'm unaware of any good currently-available browser-based readers for any of the big ebook formats. I've tried out EPUBReader for Firefox, as well as some other smaller Firefox-based reader extensions, and none of them have impressed me. I haven't tested any Chrome-based readers particularly extensively, but based on some superficial testing I don't have the sense that options are particularly great there either.

This state of affairs feels intuitively wrong to me. The browser is, in a significant sense, the natural home for EPUB-like reflowable-text ebooks, to a greater degree than it's the natural home for a great many of the other things people manage to warp it into being used for; after all, EPUBs are underlyingly made of HTML-file-trees. My own reader-in-progress will be browser-based. But nonetheless, for now, my advice for browser-based readers boils down to "don't use them unless you really need to".

If you do have to use one, EPUBReader is the best extension-based one I've encountered. I have yet to find a good non-extension-based website-based one, but am currently actively in the market for such a thing for slightly-high-context reasons I'll put in the tags.

Browser, PDF

Firefox and Chrome both have built-in PDF readers which are, like, basically functional and fine, even if not actively notably-good. I'm unaware of any browser-based PDF-reading options better than those two.

Browser, CBZ

If there exist any good options here, I'm not aware of them.

Windows, EPUB

Calibre's reader is, unfortunately, the best on the market right now. It doesn't have a very good scrolled display mode, which is a mark against it by my standards, and it's a bit slow to open books and has a general sense of background-clunkiness to its UI, but in terms of the quality with which it displays its content in paginated mode—including relatively-uncommon sorts of content that most readers get wrong, like vertical text—it's pretty unparalleled, and moreover it's got a generally wider range of features and UI-customization options than most readers offer. So overall it's my top recommendation on most axes, despite my issues with it.

There's also Sigil. I very emphatically don't actually recommend Sigil as a reader for most purposes—it's marketed as an EPUB editor, lacks various features one would want in a reader, and has a much higher-clutter UI than one would generally want in a reader—but its preview pane's display engine is even more powerful than Calibre's for certain purposes—it can successfully handle EPUBs which contain video content, for instance, which Calibre falls down on—so it can be a useful backup to have on hand for cases where Calibre's display-capabilities break down.

Windows, PDF

I use SumatraPDF and think it's pretty good. It's very much built for reading, rather than editing / formfilling / etc.; it's fast-to-launch, fast-to-load-pages, not too hard to configure to look nice on most PDFs, and generally lightweight in its UI.

When I need to do fancier things, I fall back on Adobe Reader, which is much more clunky on pretty much every axis for purposes of reading but which supports form-filling and suchlike pretty comprehensively.

(But I haven't explored this field in huge amounts of depth; plausibly there exist better options that I'm unaware of, particularly on the Adobe-reader-ish side of things. (I'd be a bit more surprised if there were something better than SumatraPDF within its niche, for Windows, and very interested in hearing about any such thing if it does exist.))

Windows, CBZ

My usual CBZ-reader for day-to-day use—which I also use for PDF-based comics, since it has various features which are better than SumatraPDF for the comic-reading use case in particular—is an ancient one called CDisplayEx which, despite its age, still manages to be a solid contender for best in its field; it's reasonably performant, it has most of the features I need (good handling of spreads, a toggle for left-to-right versus right-to-left reading, a good set of options for setting how the pages are fit into the monitor, the ability to force it forward by just one page when it's otherwise in two-page mode, et cetera), and in general it's a solid functional bit of software, at least by the standards of its field.

The reason I describe CDisplayEx as only "a solid contender for" best in its field, though, is: recently I had cause to try out YACReader, a reader I tried years ago on Windows and dismissed at the time, on Linux; and it was actually really good, like basically as good as CDisplayEx is on Windows. I haven't tried the more recent versions of YACReader on Windows directly, yet; but it seems pretty plausible that my issues with the older version are now resolved, that the modern Windows version is comparable to the Linux version, and therefore that it's on basically the same level as CDisplayEx quality-wise.

Mac, EPUB/PDF/CBZ

I don't use Mac often enough to have opinions here beyond "start with whatever cross-platform thing is good elsewhere, as a baseline, and go on from there". Don't settle for any EPUB reader on Mac worse than the Calibre one, since Calibre works on Mac. (I've heard vague good things about Apple's native one; maybe it's actually a viable option?) Don't settle for any CBZ reader on Mac worse than YACReader, since YACReader works on Mac. Et cetera. (For PDFs I don't have any advice on what to use even as baseline, unfortunately; for whatever reason, PDF readers, or at least the better ones, seem to tend not to be natively cross-platform.)

Linux, EPUB

For the most part, my advice is the same as Windows: just go with the Calibre reader (and maybe use Sigil as a backup for edge cases). However, if you, like me, prefer scrolled EPUB-reading over paginated EPUB-reading, I'd also suggest checking out Foliate; while it's less powerful than the Calibre reader overall, with fewer features and more propensity towards breaking in edge cases, it's basically functional for normal books lacking unusual/tricky formatting, and, unlike Calibre, it has an actually-good scrolled display mode.

Linux, PDF

I have yet to find any options I'm fully satisfied with here, for the "fast launch and fast rendering and functional lightweight UI" niche that I use SumatraPDF for on Windows. Among the less-good-but-still-functional options I've tried out: SumatraPDF launched via Wine takes a while to start up, but once launched it has the usual nice SumatraPDF featureset. Zathura with the MuPDF backend is very pleasantly-fast, but has a somewhat-unintuitive keyboard-centric control scheme and is hard to configure. And qpdfview offers a nice general-purpose PDF-reading UI, including being quick to launch, but its rendering backend is slower than either Sumatra's or Zathura's so it's less good for paging quickly through large/heavy PDFs.

Linux, CBZ

YACReader, as mentioned previously in the Windows section, is pretty definitively the best option I've found here, and its Linux version is a solid ~equal to CDisplayEx's Windows version. Like CDisplayEx, it's also better than more traditional PDF readers for reading PDF-based comics.

iOS/iPadOS, EPUB

My current main reading app is Marvin. However, it hasn't been updated in years, and is no longer available on the app store, so I'm currently in the process of getting ready to migrate elsewhere in anticipation of Marvin's likely permanent breakage some time in the next few years. Thus I will omit detailed discussion of Marvin and instead discuss the various other at-least-vaguely-comparably-good options on the market.

For general-purpose reading, including scrolled reading if that's your thing, Apple's first-party Books app turns out to be surprisingly good. It's not the best in terms of customization of display-style, but it's basically solidly functional, moreso than the vast majority of the apps on the market.

For reading of books with vertical text in particular, meanwhile, I use Yomu, which is literally the only reader I've encountered to date on any platform which has what I'd consider to be a sensible and high-quality way of handling scrolled reading of vertical-text-containing books. While I don't recommend it for more general purposes, due to awkward handling of EPUBs' tables of contents (namely, kind of ignoring them and doing its own alternate table-of-contents thing it thinks is better), it is extremely good for that particular niche, as well as being more generally solid-aside-from-the-TOC-thing.

iOS/iPadOS, PDF

I use GoodReader. I don't know if it's the best in the market, but it's very solidly good enough for everything I've tried to do with it thus far. It's fast; its UI is good at getting out of my way, while still packing in all the features I want as options when I go looking for them (most frequently switching between two-page-with-front-cover and two-page-without-front-cover display for a given book); also in theory it has a bunch of fancy PDF-editing features for good measure, although in practice I never use those and can't comment on their quality. But, as a reader, it's very solidly good enough for me, and I wish I could get a reader like it for desktop.

iOS/iPadOS, CBZ

YACReader has an iOS version; following the death of my former favorite comic reader for iOS (ComicRack), it's very solidly the best option I'm aware of on the market. (And honestly would be pretty competitive even if ComicRack were still around.) I recommend it here as I do on Linux.

Android, EPUB/PDF/CBZ

It's been years since I've had an Android device, and accordingly have very little substantial advice here. (I'm expecting to move back to Android for my next phone-and-maybe-also-tablet, out of general preferring-open-hardware-and-software-when-practical feelings, but it'll plausibly be a while, because Apple is much better at long-lasting hardware and software than any Android manufacturers I'm aware of.) For EPUB, I recall Moon+ reader was the best option I could find back circa 2015ish, but that's long enough ago that plausibly things have changed substantially at this point. For CBZ, both YACReader and CDisplayEx have Android versions, although I haven't tried either and so can't comment on their quality. For PDF, you're on your own; I have no memories or insights there.

Conclusion

...and that's it. If there are other major platforms on which ebook-reader software can be chosen, I'm failing to think of them currently, and this is what I've got for all platforms I have managed to think of.

In the future... well, I hope my own reader-in-development (slated for 1.0.0 release as a Firefox extension with only EPUB support, with ambitions of eventually expanding to cover other platforms and other formats) will one day join this recommendation-pile, but it's currently not yet in anything resembling a recommendable form. And I hope that there are lots of good reader-development projects in progress that I currently don't know about; but, if there are, I currently don't know about them.

So, overall, this is all I've got! I hope it's helpful.