By default firefox saves your bookmarks to

~/.mozilla/firefox/$PROFILE.default/bookmarkbackups

You can see there a bunch of *.json files.

ls   ~/.mozilla/firefox/*.default/bookmarkbackups
bookmarks-2010-07-07.json  bookmarks-2010-07-29.json  bookmarks-2010-08-03.json  bookmarks-2010-08-05.json  bookmarks-2010-08-06.json  bookmarks-2010-08-10.json

If you use more than one machine or you’ve recently changed / reinstalled your OS and you want to restore the old bookmarks, you need to import them manually:

  • backup your bookmarks files to a safe location e.g.
    cp -r ~/.mozilla/firefox/*.default/bookmarkbackups user@other_machine:~
  • on your new / other machine, open firefox then go to Organize Bookmarks
  • go to Import and Backup –> Restore and choose the most recent file from the list of *.json files that you previously copied

That’s it. You can now use your bookmarks. There are, of course, other (more elegant) solutions such as using a browser addon (e.g. Xmarks) but some people are concerned about privacy, or they just don’t want to complicate things.