It's happened to me a couple of times as a result of Firefox not exiting properly, usually stemming from a faulty extension. If it continues to be a problem I would try uninstalling lesser-known extensions and even repair Firefox.
There is good news, though: Firefox 1.5+ automatically backs up your bookmarks on a daily basis as a preventative measure in just this type of situation. Go to your Firefox Profile (see the above link for the location) and open the "bookmarkbackups" folder, which contains 5 backups. Choose the most recent one that is not corrupt (you can open the file to view the list of websites), rename it simply "bookmarks", and replace the "bookmarks" and "bookmarks.bak" files with it. (More information in the above link.)
You can also backup your bookmarks manually two ways:
1.) Navigate to your bookmarks folder and copy the bookmarks file.
2.) Go Bookmarks->Manage Bookmarks->File->Export and save a copy of your bookmarks wherever you like.
Hope this helps,
John
I don't know if anyone else has experienced this, but Firefox will randomly just delete all of my bookmarks. It will go weeks, sometimes a couple of months and be fine. Then I'll fire it up and they're gone. I then have to import them back from IE, but I've lost the ones I added since then. Is there a way to save to bookmarks to a file so that if they get wiped I can just put them back in? Better yet, is there a way to stop Firefox from doing this to begin with?
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic