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Are you using Ubuntu?
Did you install Firefox from a vanilla Firefox rather than the Ubuntu provided one?
If so then you probably followed something like this: https://linuxconfig.org/how-to-install-uninstall-and-update-firefox-on-ubuntu-18-04-bionic-beaver-linux#h6-3-1-install-firefox.
At the end of that there is a symlink, it means all of the existing logos will point to the new install of Firefox.
The catch: If you have done this, when Firefox releases a new version the system Snap version of Firefox provided by your OS will update and clobber the symlink... thus when you fire up Firefox you fire up the system version rather than the one you installed... and now you're using 2 distinct Firefox profiles... the vanilla one sync'd everywhere, and this OS reverted one you accidentally started using again.
Or: You're directly accessing the manually installed version and it isn't getting updated when the OS version one is being updated because you did it manually.
That is... this is likely if that was the kind of approach you took when installing.
Velocio-
Did you install Firefox from a vanilla Firefox rather than the Ubuntu provided one?
That's lost in the mists of time, unfortunately.
the kind of approach you took when installing.
That's a generous way of looking at the rather cavalier way that I maintain my system.
To complicate it, I also have firefox-esr installed (because pre-Quantum). I purged that, and removed the PPA.
I purged the firefox 60.x that I had, and reinstalled using aptitude, from the ubuntu repo.
It defaulted to version 58.something
I think I've fixed it now though - I did start to use the package from the firefox website, but there are so many symlinks between /opt, /etc and other places, it was getting messy.
Then I remembered that I could download the deb package that I needed and install directly from that. Where I also discovered that If I added the security repo, it would do it automatically.
tl;dr my repos were a bit messy, and adding the security repo fixed the problem.
NotThamesWater
At some point in the past, I've installed an older version of firefox than the current release.
For the life of me, though, I can't remember what I did, nor can I undo it, and having different versions across multiple devices is really messing up syncing everything.
Any ideas?