After uninstalling all international characters display wrong Rated 1 out of 5 stars

Using TB 6 on Mac OS 10.6.5 I found that after uninstalling TB Conversations all my international Characters (Umlauts, etc.) are displayed wrong. Could anybody please give me a hint how to get this fixed?

This review is for a previous version of the add-on (2.0.8). 

I'm pretty sure this is unrelated to Thunderbird Conversations. Help for Thunderbird is at