Some days ago we noticed that with the Mozmill 1.5.12 release we accidentally dropped the support for Firefox 3.6 or the Gecko 1.9.2 platform. It has been introduced with the fix for the global per compartment issue on bug 751424, especially with the addition of the ‘outer-window-destroyed’ observer notification listener.
Given the EOL of Firefox 3.6 and the Gecko 3.6 platform we do not intent to push another hotfix release for Mozmill 1.5 at this time. If you have to test applications which are based on Gecko 1.9.2 you should use Mozmill 1.5.11.
If you feel that this doesn’t solve your problem and we should re-add support for Firefox 3.6, please get in contact with us and together we can figure out what will be the best strategy for you.