Hmmm... not seeing the same behavior today. There have been upgrades in the meantime, of course, but I doubt that this was addressed. Must have been something off that day. It did run off and start downloading the header of every single message in my GMail account, which I would rather it didn't as I really only care about the mail that I have received recently (I only use Thunderbird for encrypted emails but the web interface otherwise). That is still annoying, but since I don't want to bad mouth people with false information:
Let the record show that Thunderbird no longer becomes unresponsive for me when accessing my GMail account, must have been a glitch on that day when I first set up GMail in Thunderbird.