Welcome to the avast forum.
Regular POP3 mail accounts are automatically protected by the Internet Mail provider of avast for XP Home users.
In fact avast does not know or care whether you use Thunderbird, Outlook Express, Eudora or any other standard POP3 client. avast intercepts the use of the POP3/SMTP well known mail ports that all these clients use.
There are a couple of "but ifs".
If you use a mail connection (such a GMail) that requires the use of an encrypted connection (and usually different ports) then by the definition of "encrypted" that email connection cannot be scanned by avast.
The other one is the use of third party programs (MrPostman, YPops, Hotmail Popper, FreePops, Thunderbird's WebMail extensions) that convert Webmail accounts - Hotmail, Yahoo etc - to a POP3 stream to be downloaded to Thunderbird are not scanned by default.
If you are using one of these functions then let us know and we can advise on the steps needed to have this mail scanned by avast.