Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.1.9) Gecko/20100317 Lightning/1.0b1 Thunderbird/3.0.4
It seems to happen intermittently between restarts of Thunderbird, although I can't confirm it at this stage (maybe there's been a few updates recently).
I can confirm this issue in:
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9.1.9) Gecko/20100317 Lightning/1.0b1 Thunderbird/3.0.4
It seems to happen intermittently between restarts of Thunderbird, although I can't confirm it at this stage (maybe there's been a few updates recently).