Yes, quite annoying. Another workaround could be to disable "Show recent applications in dock", but of course this is not so convenient. Go to System Settings > Desktop & Dock > Dock > Show recent applications in dock.
Killhippie, you are quite right, if I tick "Delete cookies..." the odd behavior with the extra icons is back - my test wasn't good enough. I have also tried a complete new install, after renaming "/Users/my_user_name/Library/Application Support/Firef...
You can reset your Firefox by removing two files, but remember to backup your bookmarks, before you do that - it worked for me.The files I have removed was installs.ini and profiles.ini - maybe you only need to remove one of them.The location of the ...