FAIL: Thunderbird: Replying to thread

[Rich Emberson]

I use gmail.

When I reply to a thread using Thunderbird, the response never
shows up, but, if I include the email of the person who wrote
the thread I am responding to, that person gets the email.

On the other hand, if I log in to my gmail account, the
I can post a response, that that response appears as a new,
top-level discussion thread.

How do I response to a message in a thread using Thunderbird
and have the [e-mail] accept it?

Thanks
Richard