That's happened to me a couple of times with Firefox! I don't know why. I couldn't get them back the first time either.  The second time I was lucky enough that I had saved a copy of my bookmarks .htm file so managed to get most of them back.

Change your settings to view hidden files.  Then go to your profile and find Application Data.  Look for Mozilla  and look inside. See if there is a backup of your bookmarks .htm file.  Mine seem to back themselves up.  If you have a backup in there, change the name to  bookmarks.htm  and put it into the main Mozilla folder and your bookmarks should be back.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi