There's no 3.6.18, so there appears to be a glitch in the update checking. Or the message is not really from Firefox (malware site).

You have a corrupted user agent that identifies you as Firefox/3.5.2 if you really have Firefox 3.6.17 (or you posted with another computer). *Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.1.2) Gecko/20090729 Firefox/3.5.2 CouponCabinToolbar See: *https://support.mozilla.com/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser * http://kb.mozillazine.org/Resetting_your_useragent_string_to_its_compiled-in_default ---- You may get an offer to update to Firefox 4.0.1 If you have problems with updating then easiest is to download the full version and trash the currently installed version to do a clean install of the new version. Download a new copy of the Firefox program and save the DMG file to the desktop * Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html * Firefox 3.6.x: http://www.mozilla.com/en-US/firefox/all-older.html * Trash the current Firefox application to do a clean (re-)install * Install the new version that you have downloaded Your profile data is stored elsewhere in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder], so you won't lose your bookmarks and other personal data.