Firefox 4 is still in a beta stage and as such, many of the addons people use still aren't compatible with Firefox. Also, Firefox 4 beta is installed, by default, to a different folder, which also makes the separation between your Firefox 3 addons and Firefox 4 addons. As such, i recommend waiting for your addons to be updated and use Firefox 3 for the time being.

There are many extensions that no longer work in the latest Minefield nightly builds and Firefox 4.0b1 preview because of changes made to registering XPCOM components. See http://forums.mozillazine.org/viewtopic.php?f=23&t=1937759 ..... http://blog.mozilla.com/addons/2010/06/14/multiple-breaking-changes-are-coming-for-components-in-extensions/