Does it require a Firefox account? Does it only store them locally if you haven't signed in to Firefox? This is the sort of failure I've seen, where people think their passwords are synced but because they didn't sign in years ago it's actually not backed up at all. At least on Chrome you get reminded of that all the time on YouTube/Google search, etc.

I know for Safari all the sync is via iCloud meaning if you're not signed in it's locally stored and vulnerable in that way. Especially as many people can't/don't sign in to their own iCloud on work computers, or don't have a Mac.