I've been running a previous version of Firefox 3.x without incident and just tried upgrading to Firefox 6. I've had this same problem with trying to upgrade to Firefox 5 in the past. The installer runs fine and reports no problems. The first time I try to run Firefox after installing, I get a dialog box saying something along the lines of "Checking your add-ons for compatibility..." and the application IMMEDIATELY hangs. I have to kill the process from Windows Task Manager. Afterwards, trying to run Firefox simply results in no visible instance of anything running, although the process still hangs in the background taking up 25% CPU and ~10-20MB of memory. However, when I try opening a locally saved .html file, Firefox successfully runs and initially loads/displays the page, THEN hangs. I've closed as many background applications as possible and still get the same problem.