According to
T-Bird FAQs it says,
"Is Thunderbird susceptible to e-mail viruses?
Thunderbird will not allow a virus or worm to execute automatically. You can see what attachments have been sent to you without a virus being able to execute, and you would have to save a file to your system and deliberately run it before it could cause any harm.
JavaScript is switched off by default for mail and news, so an e-mail cannot run script code just by being opened.
As with any mail program, take proper caution before running any file that you receive in e-mail. Appropriate anti-virus software should also help keep you safer."
I haven't gotten a virus so I don't know if it works. But the fear of a virus is why I switched from Outlook Express to T-Bird.