Bookmarks are stored in:
C:\Users\USERNAME\AppData\Roaming\Mozilla\Firefox\Profiles\XXXXXXXX\places.sqlite
In the same folder, you will see a bookmark backups folder. Inside there, you will see backups of your bookmarks that can be imported back into firefox.
Also, from the bookmarks screen, there is a save/load bookmarls option. I've also found firefox's sync to come in handy, even with just one computer. After reinstalling firefox, just use your sync password and it will sysnc your bookmarks, effectively restoring them.