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 ''Several people have reported that you can skip this step:'' 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!