Joachim,
the problem exists *not with the 2.0.0* version but with *mailredirect-0.7.4-tb3-20100708.xpi* , available from http://downloads.mozdev.org/mailredirect/mailredirect-0.7.4-tb3-20100708.xpi.
Yes, I got that. My FYI was just a general one since it seemed that "g" may have been suggesting it possible to run the older version on TB3. Just dotting the "i"s and crossing the "t"s. And being annoyingly pedantic. :-) :-)