We also had this on several machines in our company. It occoured by updating Thunderbird from .21 to .22.
Now we found than you must set the standard programms in 'software' new.
We are in germany, so it is hard for me to explain it for the english windows.
In German it is:
"Start - Einstellungen - Systemsteuerung - Software"
In English version it might be:
"Start - settings - systen - software"
And then under the lower left button "Programmzugriff und Standards festlegen"
There you must select 'user settings' and set the standard mailprogramm toThunderbird.
Now it works.
Hopefully you can find it with my strange english...
I'm not sure if this is where you mean...but when I go to Start > Settings > Control Panel > Internet > Programs, Mozilla Thunderbird is already the selected email program. Is there somewhere else I should check?
No, it isn't in Start > Settings > Control Panel > Internet >
it is in Start > Settings > Control Panel > Software (where you can uninstall your installed programms)
And then klick in the left pane the lower button.
Name might be something like 'Programm access and standards'.
Wow -- I have never before in my life seen that screen (Control Panel > Add or Remove Programs > Set Program Access and Defaults), and Thunderbird isn't even listed under the 'custom' option.
Anyhow, I have found some threads where others are reporting the same problem with the Thunderbird 18.104.22.168 upgrade a couple weeks ago. Time to revert. Thanks, all, for your input.