This may be due to a way in which Avast is configured. I don't use it so I cannot be specific but I have had problems in the past with antivirus software causing applications to run slowly or not at all. Avast may be also interfering with extensions if it determines (probably incorrectly) that an extension is displaying unusual behaviour. The Avast icon spinning suggests that Avast is delaying the launch of Firefox by performing some function on the Firefox application as it is launching which is delaying the launch. Until someone can post here more specific answers, may I suggest: 1) checking in the Avast forums if there are similar problems reported there (there may be a solution posted) 2) making sure that Avast and all your add-ons are updated to the latest versions (in case a fix has been released) 3) follow the instructions at [http://support.mozilla.com/en-US/kb/Troubleshooting+extensions+and+themes Troubleshooting extensions and themes] to see if an add-on is causing the problem. 4) Temporarily disable Avast or switch to another antivirus program to see if Avast is the source of the problem rather than Firefox. [http://support.mozilla.com/en-US/kb/Firefox+hangs#Hang_at_exit Firefox+hangs#Hang_at_exit] suggests Avast causes problems when used with Zone Alarm as firewall. It may be causing similar problems for you whether or not you have Zone Alarm. I hope this helps in some way.