Version 4.0.1 has serious security issues that were fixed in Firefox 5. It would be more secure to roll back to 3.6.18. Regarding McAfee, an update for Site Advisor is expected next month, according to this recent thread: [https://support.mozilla.com/en-US/questions/840209 What happened to the Mcafee Site Advisor?]. --- 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 ( For other versions, see the ftp server at ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/ ) You could install Firefox 3.6 over Firefox 5 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. Note: I haven't actually tried this myself!