Userscripts and everything, in Firefox.
I think it crashed/shut down mid way through a sync, and lost all the addon data. When I started it up today, it acted as if everything were just being installed, but already installed. Firefox briefly acted like it was also just installed, until I did a startup cache clear.
This apparently wiped out my addon data for some fucking reason. No idea why.
No profile recovery, no session recovery, none of my extensions have their settings synced with other copies of firefox. None of it helped. No searching I've done has found ANYTHING like this.
I'm grateful it wasn't my fucking bookmarks, or my cookies, or my password manager, or whatever, but holy shit.
If you think you've got something digital, that is UNSHAKEABLE and has never had a problem before?
BACK IT UP NOW.
Figure it out. Profiles, settings - make a copy RIGHT NOW. Do it REGULARLY. This could have been SO MUCH WORSE than just my adblocking preferences and extension settings, even though there's a LOT to go through still.
Fuck.