Comment 9 for bug 933951

Actually, this version fixes it:

thunderbird (11.0~b2+build2-0ubuntu3) precise; urgency=low

  * Add a missing file which caused Lightning to break (LP: #933951)
  * Trick the addon manager in to thinking that Lightning has been updated
    when upgrading
    - update debian/thunderbird.postinst.in

 -- Chris Coulson <email address hidden> Fri, 17 Feb 2012 09:09:27 +0000

But the fact this broke so badly (and doesn't recover without reinstalling lightning) actually appears to be a lightning bug.