Firefox still doesn’t support Adobe Flash, it shows checkered boxes for parts of a web page that haven’t fully loaded, and when you zoom in and out, there’s always a second or two when text looks fuzzy before it comes into focus.
That said, Firefox Mobile has come a long way since the first beta hit mobile devices. The browser is almost fast and stable enough to consider using as a primary web browser. It also supports third party add-ons, and there are about 200 available at the moment. CPU usage has been reduced, and it’s easier to set up synchronization between the desktop and mobile versions of Firefox.
You can find more details in the release notes. Firefox 4 beta for Android is available from the Android Market or from the Firefox Mobile web site. You can also grab the Maemo version from the Firefox site.