If you're looking simply to restore the original default settings, and your Firefox profile is located in your home directory, under the hidden subdirectory ".mozilla" then you could try the following (after closing out Firefox):
(note there is, indeed, a space in-between .mozilla and .mozilla.old)
user@machine:~$ mv .mozilla .mozilla.old
This effectively forces Firefox to recreate a profile from scratch. This means no bookmarks nor any other non-default settings will remain--which sounds like it's exactly the configuration you're seeking.
Note that you would perform this command at the top of your home directory. If in doubt, "cd" by itself should take you to the top of your home directory.
Hope this helps.