I'm not running Thunderbird here right now, but...

Word merge to e-mail expects to use the MS MAPI standard. Precisely
which version of the standard is less clear. Also, if you are trying to
send HTML-format emails, Word tries to use a specific MAPI interface
that is as far as I know only available in the full Outlook (not even
Outlook Express).

As I understand it, Thunderbird 1.5 (don't know about earlier versions)
implements a Simple MAPI interface, and that interface would typically
be enabled if you have set Thunderbird to be your default mail
application. If you were using IE, you would probably do that via IE
Tools|Internet Options|Programs, otherwise I leave it up to you).

if you have done all that stuff and perhaps done anything else that
Thunderbird requires you to do to enable MAPI, and Word still cannot
e-mail to Thunderbird, you can always check out

http://tips.pjmsn.me.uk/t0002.htm

However, I have a sinking feeling that if you've done all that stuff and
it still doesn't work, it may not be possible.

Peter Jamieson