Mozilla Firefox 12.0 and Thunderbird 12.0 have been pushed out and I updated to both yesterday.
They are working very well so far.
No problems with any of the Extensions I use with Firefox 12.0 or Thunderbird 12.0, however Lightning 1.3 was updated to Lightning 1.4 during the install when it checked for updates to extensions.
All going quite well. But I was careful too. I backed up my bookmarks .json for Firefox and copied my entire Thunderbird profile just to be sure.
But it all went very well!