Mozilla has released the fourth (there was no Firefox 3.6.1 or 3.6.5) update for Firefox 3.6.6 on June 26. This update addressed issues with new plugin handler causing issues with the immensely popular Facebook game Farmville. See bug 574905 for full details.
Firefox 3.6.6 modifies the crash protection feature to increase the amount of time that plugins are allowed to be non-responsive before being terminated.
Users should be prompted soon to update, but can obtain this update via Help the Menu and selecting Check For Updates…For complete list of changes and more information, please see the Firefox 3.6.6 Release Notes.
Since this was a ‘firedrill release’ all updates that were part of the original 3.6.6 update have been moved to Firefox 3.6.7 scheduled for around the end of July. Also note this update is only for Firefox 3.6, Firefox 3.5 is still currently at 3.5.10.