I get that from using bugzilla from time to time since around 2005 maybe. Last time and one of the worst was the ALSA debacle. IIRC first time was about integration in KDE.
To recap the ALSA one, mozilla pretended ALSA had shortcomings it actually does not have (some things about edge cases of netflix and 5.1 audio or something) and it turned out it was Mozilla implementation that was so bad and pretty much abandone that no one at mozilla wanted to work on it.
It was taunted at people complaining that for ALSA to stay they should do it themselves or find someone to do it, until someone came forward and offered to do it and suddenly it was too late so it was not happening. The other reason invoked was that a very little number of people did not have libpulse installed which it turned out was false (because most distro disabled mozilla's spying) and irrelevant (libpulse is often pulled as a dependency on ALSA only system). So suddenly it was the fault of distro packagers and people caring about their privacy had it coming because they took the extra step of disabling telemetry (which is not supposed to be enable on release channel anyways).
I supposed those bug reports are still somewhere in bugzilla, if you wanna look for them. The KDE integration one is one of the longstanding open ones, and openSUSE provided a patch for better KDE integration at some point but it was refused for some reason. you can find it on AUR https://aur.archlinux.org/packages/firefox-kde-opensuse/
Even a simple one related to the ALSA debacle was ignored, see they forgot to mention dropping ALSA in the release notes somehow they overlooked this "detail" but they thought to include a warning message with a link to an URL that happen to be broken. A bug report was filed to report this along with the correct URL but the fix that should have taken about a minute to implement had not seen the light of day after two weeks and AFAIK did not happen at all.
I sincerely doubt KDE, debian, arch, ALSA are obscure platforms, and turns out chromium supports alsa (IINM firefox is the only linux browser with a hard dependency on pulseaudio).
To be honest with time I have learned not to expect better of Mozilla or more like to expect mozilla to not deliver on its marketing promises and to dismiss user feedback.
/edit
here's the recent iteration of the KDE integration bug, this one has been opened for 16 years: https://bugzilla.mozilla.org/show_bug.cgi?id=140751
here's the main one for dropping ALSA and making pulseaudio a hard dependency, it got locked and discussion continued on google groups somewhere: https://bugzilla.mozilla.org/show_bug.cgi?id=1247056
there are other with several duplicates such as: https://bugzilla.mozilla.org/show_bug.cgi?id=1345661
I can't find the main broken URL one with the duplicates, but it seems this one is about the same issue and had been marked as solved (Only took 3 months to fix a broken URL which was the only was to get details on why the browser suddenly stopped playing sound):
https://bugzilla.mozilla.org/show_bug.cgi?id=1345439
To my knowledge there are many other examples in bugzilla in the last 10 years, maybe I've been unlucky and found myself in the worst of bugzilla making my experience not representative of the actual thing, but from what I gathered around on mozillazine, HN, distro forums and some other places my experience is not out of touch with what others have to say about theirs.