As he says.
I have a backup utility on this PC (it's been there for yonks, and I've forgotten the name if I ever knew it). It backs up my bookmarks, stored passwords, and stuff. It "just works". I think it may even be a native Firefox feature, because it appears on the Tools - Options - Sync tab.
There's a separate app called Xmarks that syncs the bookmarks with my laptop. That would do more, I think, but I don't like releasing passwords to the outside world (not even the "unimportant" ones).
On the PC there's a folder under Firefox called "bookmarkbackups" with filenames like bookmarks-2015-01-15_524_-YcZCRCoiA9XzuJb2LOJgQ==.jsonlz4 which I think is where the backup facility puts them on a daily basis.
That gets backed up to a sequence of external hard drives every week, so if the PC has a major hiccup I can go back a couple of months or so. I've not needed to in years, but that's the joy of backups.
I've not "lost" a bookmark (or not noticed, anyway) in a long time.
There was for a time a bright red thing called "LastPass" that did all the above in a more snazzy way, but it started pestering me to do things I didn't want to do, so I removed it. I think it was an upmarket Xmarks.