Rated 5 out of 5 stars

it works fine on Firefox 6.0.2 but not so fine on Thunderbird 6.0.2

This review is for a previous version of the add-on (1.2.2).  This user has a previous review of this add-on.

You're right. More Tools Menu version 1.2.2 doesn't work with newer Thunderbird versions.
A new version 1.2.3 is being prepared to resolve that problem.
Thank you very much for reporting it.