Generally, when programs freeze it's because your computer can't handle all the tasks it's trying to perform at once. It could be due to too many other things happening when you're trying to run Thunderbird. I'd suggest taking a look at task manager to see if there's a lot of junk processes going on in the background. Do you have lots of other programs running? Quick way to try getting more information might be to create a new local user on your computer, then using that user to run Thunderbird and sign in to your email. If Thunderbird works fine with that new user, then the problem may lie in the way you use your machine under your regular user account.

What is your anti-virus software?