Never seen that with Thunderbird. SeaMonkey and Firefox, yes.
Mozilla apps are basically a mixture of xul, dom and JavaScript so it's not that weird to get an error like that. Thunderbird is a pig here, just bringing it to the foreground can peg both cores for seconds, wish I knew what to do about it.
What version are you using? Looking at the latest, your error seems to correspond to a progress meter, which doesn't sound right.
Do you have extensions installed? Might want to try safe mode to see if it makes any difference.