Firefox is not listed on my MBP or my wife's Vista PC...only Safari and Internet Explorer.
One way around it is to use Xmarks, which synchronises your Bookmarks/Favorites up to a central server.
If you work primarily in Firefox, just open Internet Explorer once in a while and let it sync with all of your Firefox bookmarks, then when you sync your iPhone, everything is copied across
http://www.xmarks.com/