The fix for this is the same as for Firefox 5.
Go to this post and follow the instructions.
You can install Mr. Tech Toolkit in Thunderbird too.
Exception to the instructions - when you come to adjust the MaxVersion, change it for where it says Thunderbird this time instead of Firefox - it is most likely at 3.2pre or something similar - change this value to
With this updated Mr Tech Toolkit installed in Thunderbird your old add-ons should work. Note, for some they might work only be very buggy, so use with caution. If you find one that is very buggy, disable it and restart Thunderbird.