Hey Luth,
an installation of a younger version over the old one should actually preserve your settings, bookmarks etc., though of course not the list of recently visited pages. To go sure you might want to copy the bookmarks to a safe place. They should be found in a file in a hidden directory whose name I don't know because I don't use FF, but most probably it's ".firefox" if I had to guess.
Deleting your cache might suffice, though.
Cheers!
Markus