Update 2021-12-15 as noted here, so happy this is working again!!!! FF 68.11.0.
I will say that BWC can be interpreted liberally. If it was not possible to reinstate the same functionality, say for Inbox and Achievements (expanded drop down), a simple fallback to open to the user profile page, inbox tab is a better UX than nothing/breaking.
Whatever changes were pushed today (2021-12-08 seem to have rendered a significant portion of the UI inoperable on Firefox for Android 68.11.0. Yes, that's a rather old version, but it's the last one before Firefox pushed their UI overhaul that everyone hates.
It's a similar list of issues as @dbc described for SeaMonkey
- can't comment
- can't expand Inbox, Achievements or the hamburger
- can't vote up or down
- probably more too.
I am 100% sure reporting this is not going to change anything and will just be one more of those sacrificial changes in the name of cosmetically keeping up with the Joneses taking away from the user experience, but it's documented now.
UPDATE: Interestingly enough, they did a half-baked job of breaking the UI. If I "Request Desktop site", then the Inbox and Achievements work as expected, but the hamburger and the ability to "Add a Comment" remain broken. I thought the only thing "Request Desktop site" was supposed to change was how the site presented itself, not actually change the behaviour. If it works on one mode, why not the other?
We have a third-party folder in the repo. It contains (very old versions of) a variety of libraries. Some of those libraries have multiple versions in subfolders. Some of those libraries have had ad-hoc edits made to them. It's difficult to upgrade dependencies and it's difficult to manage transitive dependencies.
Rather than tweaking/breaking a functional UI, then perhaps they'd break less of the UI and have better BWC! Makes me shudder!! Duplicating code, checking in libraries, making custom local fixes (hacks/workarounds) instead of contributing PRs back to the original ... Those were bad practices back in the 1990's!!
I'd rather have websites that work instead of ones are "hip", especially when it's supposed to be one dedicated to sharing knowledge and not marketing.
Finally, to those who are suggesting this is a possible duplicate of the exact question I referenced by link, I am not sure what to say. SeaMonkey is a different platform entirely to Android for Mobile. It is possible some, most or all of the symptoms are the same, but I can't verify that. Since it's a different app and platform, I'd say it's not a duplicate.