The version you have installed may likely be a package build installed in your packages manager as updates are provided there. If you are not getting anymore Firefox package updates then perhaps your version of Linux distro is eol with no more package updates. Mozilla does not provide internal Firefox updates to third-party builds.

The Firefox tarball from Mozilla say https://www.mozilla.org/firefox/all/desktop-release/ is just an archive. There is no package or installer setup like you get on Windows. There was a Linux installer of sorts long ago in early days but it was an extra step (and more confusing for the less technical users) for the same end result of the tarball as to why they discontinued it.

The basics is you extract the tarball then run the firefox script or firefox-bin to start Firefox. You can create desktop or such launcher shortcut and there are Firefox icons in the firefox/browser/chrome/icons/default/ folder. The chrome in this case has nothing to do with the Google Chrome browser. Oh and make sure the firefox folder has read/writer permissions for the user to get internal firefox updates from Mozilla otherwise you will have to do so say as root. An popular place is to have firefox folder in /home/~

Thank you so much for this info, this will permit me to address my own personal issue and I'll be back in business. I appreciate it.

However, I think my larger question still stands. Why is it this hard (and undocumented)? I'm glad to know that the reason the Firefox Updates panel is "blanked out" for me, is probably due to this coming from a distribution package (Debian), as I'm sure it did. But this begs a few questions: why can't there be at least some text in the panel, basically the same as what you wrote above, explaining the situation, instead of just rendering it inactive without notice? And why isn't this covered anywhere in the Support documentation? It must be an extremely common issue, at least for Linux users.

I bet there are hundreds or thousands of Linux users out there who have hit the same frustrations that I did, but just said "F it" and chose not to do anything further. Thus, come March 14th, I guess they'll either have browsers that stop working and they'll probably move to some other alternative in annoyance; or worse yet, the browsers will still work to some extent but they'll be subject to (I guess?) some security vulnerability that has been judged to be so severe that the "upgrade before March 14th" banner has been forcibly placed in front of everyone. This attitude of "we're not responsible for third-party aggregators" isn't wise or responsible, considering that Firefox/Mozilla is de facto one of the most important and common pieces of software in the free software world. IMHO.

The lowest amount of effort that should be expended on this would be at least to include the necessary explanations and workarounds in the Support documentation. Just a link to this very thread would even do it. So I'm not clicking "Solved the problem", even though I'm sure you'll close out this issue as "Solved", because I consider this to be an ongoing bug. Please fix! And, thanks again for the info.

--Benjamin Ketcham-McGrath

The March 14 date does not mean the Firefox versions not 115.13.0esr or newer or 128.0 or newer will just stop working. The biggest change people will notice is any extensions then have installed will not work.

https://support.mozilla.org/en-US/kb/root-certificate-expiration