One of the reasons that FireFox is specifically not yet supported is that FireFox does not yet support HTML5 behaviors very well, and FM is moving to HTML5. (The new FMServer admin interface is HTML5.)
I just used this url
to test one version of the FireFox browser, and it got a score of only 68 out 555 points for FireFox's support of HTML5 !
I should have known it was that simple. THX.