Early in May 2012 Mozilla will enable the auto update of the 3.6 version and force it to upgrade to the newer version 12 for greater security. Mozilla issued the final security update for 3.6 on January the 31st 2012 and also ended support for it last week (April 24th). The browser itself has been \"officially\" retired and automatic upgrading is being switched on, the date is not yet announced, but it will happen soon.
If you want to keep the older Firefox 3.6, then you will have to select to disable updates before the May auto-upgrade. In Windows, you access the setting via the Options from the Firefox menu, then Options again (on the Mac, Preferences from the Firefox menu), click the Updates tab, then choose \"Never check for updates.\"
If you want the auto-update then leave everything as it is, Mozilla however in a recent statement said \"we strongly advise our users to upgrade from Firefox 3.6, as they will no longer receive critical security updates.\"
If you use Firefox 3.6 then you should soon see a notice that tells you the browser will automatically be upgraded to Firefox 12.
Much of the reasoning behind the move is to improve security and take into account the moves made toward incorporating HTML5 and the changes required for IPV6 which is also coming soon.