Is there a possibility that it’s an issue with the desktop Firefox? Maybe it’s sending “garbage” to Sync due to profile corruption, etc. The passwords in your desktop Firefox work fine? Maybe try clearing its cache, if you haven’t already, or go nuclear by creating a new profile, then set it up as you want, and adding your passwords, then using that new profile to see if you can sync your passwords, etc. Sorry, but other than that drastic action, I’m out of ideas.
One other thing, do your browser history and bookmarks sync?
Well, yeah, I get that, even if I don’t find those episodes bad myself, but I do think episode two, Bojack Hates the Troops, is a really good satirical episode that shouldn’t be missed.