After I updated Thunderbird to 12.0.1, it claimed that 3 of my themes were incompatible. This did not seem likely, because I had bumped up the max version for the target app in the install.rdf file for each of them. I was able to reinstall them after disabling compatibility checking, and they worked fine. I also decided to get the latest version of the MR Tech Toolkit beta extension. Thunderbird refused to use it, claiming that it was incompatible. I thought that the newest versions of Mozilla applications made extensions compatible by default. Why make an exception for this one extension? After bumping up the max version for Toolkit, Tbird used it without complaining. Then just to see what would happen, I clicked the "Enable" button next to the "Compatibility checking is disabled" warning, and everything continued to work, but after closing and reopening the window again, the warning was back. Why? If I enable something, why tell me that it is disabled after I reopen the window?
I turned off the computer overnight, and started it up again the next day, launched Tbird, looked in the Addons window, everything is working, and the compatibility checking warning is gone. That makes me wonder - what is Thunderbird 12.0.1 doing? First, it tells me that something that is supposed to be compatible is not, then it refuses to believe that compatibility checking is re-enabled until after I shut down the computer and then restart it.
UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:12.0) Gecko/20100101 Firefox/12.0