Comment 15 for bug 12097

I can confirm this, but I don't know how to change the status of the bug to confirmed on bugzilla. I have the exact same issue on Ubuntu 9.10 using Firefox 3.5.5 and Thunderbird 2.0.0.23.

This seems to be a recurring issue which gets fixed and then regresses to old behavior:
https://bugs.edge.launchpad.net/ubuntu/+source/mozilla-thunderbird/+bug/12097

I think it also may have to do with mishandled upgrades. I think creating a new profile, or using the following workaround may work. https://bugs.edge.launchpad.net/ubuntu/+source/firefox-3.0/+bug/222944/comments/3