I believe this is the original ticket that requested implementation of a tray icon in Linux: https://bugzilla.mozilla.org/show_bug.cgi?id=18732. It was requested 27 years ago and closed 2 years ago.
And it still doesn't work properly (for me it didn't work at all). And this is the current (still open) ticket https://bugzilla.mozilla.org/show_bug.cgi?id=1942125