I recently upgraded from Firefox 3.6 to Firefox 4 and it's noticeably slower. Also, when I go to some websites, certain components of the site won't load. For example, when I go to a Blogger blog, the "followers" section will not load. Not much of an upgrade if you ask me, so now I'm going to uninstall and attempt to get version 3.6 back. As of now I would not recommend upgrading if you use Mozilla Firefox as your browser.