Well, this should work. But I'm running Firefox 3.6.13 on Gentoo and 4.0b8 prerelease on Slackware and everything is working. Can't it be possible, that your upgrade 3.6.4 pre is somewhat outdated? maybe that's the real problem. Isn't there a more recent version available for you distribution? (but I don't know exactly the differences between firefox and namoroka, maybe they have different releases)

Markus