In Firefox (I'm using 8.0 on Linux and the Win version may be slightly different) from the menu:
History>Show All History (keystroke Ctrl-Shift-H) gives a new window with an Import & Backup item. The options for Bookmarks include Backup and to HTML and Restore.

Another approach - Firefox keeps everything in a profile folder, with a name like aj86k0dg.default. In Linux it's located in your home/.mozilla/.firefox folder - I don't know where Windows hides it. The profile folder contains a bookmarkbackups folder with a daily record of your bookmarks.

I don't know how much this grows - I have 10 days history there, which corresponds to my fresh installation of this version. I also don't know how to convert the information to something usable, but it must be possible.

Phil