It currently works in three browsers, IE, Mozilla (not firefox) and Netscape. However, with IE7 and above in Vista it doesn't work because of the way IE works in protected mode. So it is somewhat limited in that area, but it also has a use on your system not just the web.
At one point it was considered to drop it, but I think that has passed and will continue in the Pro version I can't recall from all of the posts in the beta sub-forum if it is possibly going to be in the avast antivirus free 5.0, you will have to do some digging there.