This is a critical bug in Firefox. I've had it happen to me, and so have others. Your bookmarks just go away. Your bookmarks are lost. It seems a lot of people are willing to chalk it up to user error, which is really annoying, because sometimes they do in fact, just disappear.

If you got here because this has happened to you, here is a list of things to do:

1. Make sure you aren't running 2 firefoxes under a different profile.. Check this post here for some information
2. do a search for bookmarks.html on your system. Make sure you DO search hidden files! Open them up with your browser, and if it is your bookmarks, then you are safe. Away you go!
3. Do a disk-repair and check. your bookmarks might be in a file called fileXXXX.chk (where XXXX is a 4 digit number, like 0001). What you should do is open the file with a text editor, (notepad if you have to), and you are looking for HTML, with a list of all your bookmarks.. (good luck).
4. Use an undelete utility, and look for the file bookmarks.html.moztmp. Apparently that might be your bookmarks file, but it may take some massaging.

Regardless of whether or not his has happened to you... remember

• Start backing up! learn from your mistakes. Bookmark Backup is pretty cut and dry, but there are other solutions out there.
• This is bad, this is a very big, bad scary bug with Firefox. But the grass probably isn't all that greener on the other side. IE is still not terribly secure, and, depending on your system, can crash if you even look at it wrong. I don't know about Opera.
• There has been some discussion on bugzilla about this problem, so it will hopefully be addressed and fixed.