Was a big FF user and Firefox4 looks fantastic. I explored supporting FF and it would be drop dead simple except for one killer issue, cross site XHR calls. I have, as of yet, not found a solution to this problem on Firefox that does not involve the user munging around in low level security settings and flags. And I won't ask a user to do that. The web is no place to click around in with compromised security.
With FF4 out I'll take a hard look at it again.