I have been using Firefox for WIn32 (just automatically updated to 1.5.0.6) on my Suse 10.1 system with XO 5.0.3 for the last few weeks without negative incident. It works much bettter that IE in XO, by the way. I use Firefox for WIin32 for viewing multimedia sites, especially listening to baseball games via MLB.COM, because there are some unsolved issues with getting all the necessary Windows multimedia plugins to work through native Firefox.
I give it a 5 rating.