Hi. I have the same problem about ff not taking into account the installed langpack each time there is a ff upgrade. I just switched from ff77 to ff78 and the interface switched back to english so I had to manually select and install my language from the preferences page.
That's when I saw this "fixed" bug. It doesn't seems fixed in my point of view because when I opened ff it was again in english. Reading the last message in the ticket, I started ff with a new profile and indeed the new profile opened in my language. There are 3 observations that I could make at the moment : 1. first there were 3 profiles : - mine (default) - another (default_release) - the newly created (tests) 2. that behavior (re-installing locales after each upgrade) appeared when I switched from ff-esr to ff 3. it seams that my default profile (holding my configuration and everything I need for day-to-day use) is not linked with the system's langpack, but only the one I install from the preferences. Is there a mean to connect my profile with the system l10n package instead of the one installed from the preferences page ?