Before you go compiling a new kernel mod for the ATi video card. You may thank
about making a backup you bookmarks from firefox.

Try to sudo apt-get -f install "firefox"
else
sudo apt-get purg "firefox"
rm -rf ~/.mozilla
sudo apt-get install "firefox"

This may or may not help :|

You can change default apps and other stuff thou control center in gnome. just
run "gnome-control-center" from the console.