This does not appear to be an ubuntu specific issue: https://wiki.mozilla.org/Thunderbird:Testing:Shutdown_Hang and the linked https://bugzilla.mozilla.org/show_bug.cgi?id=532435 are possible culprits.
** Bug watch added: Mozilla Bugzilla #532435 https://bugzilla.mozilla.org/show_bug.cgi?id=532435 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/612907 Title: 100% CPU usage on exit -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
