[ubuntu] Why does Firefox updates make it freeze

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http://ubuntuforums.org – Since june 2010 has every new update of Firefox caused it to freeze immediately after launching it, no window shows but it's there in the list of sleeping processes. Trying to launch it a second time results in the message "Firefox is already running". The workaround is to remove the .Mozilla folder in my home directory, then Firefox works until the next update, but of course I have to reenter all settings and add-ons. Anybody know why this occurs? I now run Firefox 3.6.10 with Xmarks and Downthemall add-ons. (HowTos)