Do you use a downloader add-on? It might not be fully compatible with Fx4. You could try Firefox's Safe Mode to diagnose add-on issues. I recommend first making a backup of your Firefox settings: [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information]. Next, try starting Firefox in Firefox [http://support.mozilla.com/kb/Safe+Mode Safe Mode]. Be careful not to "reset" anything permanently if you didn't back up. Does downloading work correctly? If so, most likely it's a problem add-on. --- If you are not interested in troubleshooting, here's the process to roll back: First, I recommend backing up your Firefox settings in case something goes wrong. See [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information]. Next, download and save Firefox 3.6 to your desktop for future installation. http://www.mozilla.com/firefox/all-older Close Firefox 4, and uninstall it. Do not remove your personal data and settings, just the program. Then finally you can reinstall Firefox 3.6. Unless you have installed an incompatible add-on, it should pick up where you left off. If there are serious issues, please post back with details. Note: I haven't actually tried this myself!