Are you running thunderbird from within KDE without thunderbird-gnome-support installed? Thunderbird has no KDE integration, but it should probably still open x-www-browser (which most likely isn't your default browser anyway).
If you're running in KDE, then this is really a duplicate of bug 543064
Are you running thunderbird from within KDE without thunderbird- gnome-support installed? Thunderbird has no KDE integration, but it should probably still open x-www-browser (which most likely isn't your default browser anyway).
If you're running in KDE, then this is really a duplicate of bug 543064