Cause: Automatically updating to Firefox 18.104.22.168.
Effect: Nuked all my bookmarks. Wiped all my settings for both the browser and all extensions. Any attempts to change settings or set bookmarks fail.
Solution: Start Firefox in Safe Mode, Select “Reset toolbars and controls” and press “Make Changes and Restart”
Effect: Everything is back to normal except the bookmarks are still trashed so I have to recollect them, but I’m taking the opportunity to organise them correctly this time: