FF4 is already faster and more stable than the 3.6 version. Also, if everyone would use stable software, who would download Avast public beta?
Anyhow I made an XPI from "c:\Program Files\AVAST Software\Avast\WebRep\FF" and installed it into Minefield: the button is visible, but no ratings queried or sent. Also the rating window is broken.
You could make a beta for the Firefox extension too, it seems FF4 will be here soon - and till then the Avast GUI should say "not supported" instead of "installed" for FF4.