As for the firefox issue, just download the official binaries from mozilla.com, stick them in /opt, chmod them to your user and enjoy.
Keep the xorg.conf if you have any custom tweaks.
My backup policy for my machines is to backup /etc, any useful scripts in /usr/bin, anything worth keeping in /opt and thats about it. I already have backups of things like skins for gkrellm, xmms, mplayer, etc. If you have any custom backgrounds in /'usr/share/backgrounds, remember to tar those as well.
As long as you seperated /home partition in the first place, there shouldnt be a problem as you can rewrite your / partition with the new distro and be back on your feet within an hour or so, even less if you write a script to automate the installation of your favourite apps and utils.
Nowadays i run a dev directory within my users home, and i stick all my favourite apps, utils and games in there, so anything i personally use is always going to be available to me, and wont require the same level of backups as i normally do.