Due to security vulnerabilities, rolling back to Firefox 4.0.1 is not recommended. Firefox 3.6.18 would be the safer choice. 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]. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.) Next, download and save Firefox 3.6 to your desktop for future installation. http://www.mozilla.com/firefox/all-older Close Firefox 5. You could install Firefox 3.6 over it (many have reported success) or you could uninstall Firefox first. If you uninstall, do not remove your personal data and settings, just the program. Unless you have installed an incompatible add-on, Firefox 3.6 should pick up where you left off. If there are serious issues, please post back with details.