Hi Peter,
Sorry for my mistake.
It is a known issue on Firefox and Windows Live Messenger but it is fixed. Both Firefox and Thunderbird are developed by Mozilla so I made a confusion here.
I tried to install Thunderbird (2.0.0.22 and upgraded automatically to 2.0.0.23). However I cannot reproduce the issue.
Please let me know if there is any setting applied on these 2 programs such as run in Compatibility Mode.
Meanwhile please disable all other third party applications and try to test the issue again. You can try a Clean Boot. Please refer the steps for Windows Vista.