Summary
Emails sent from a Thunderbird identity I retain the 'from' address of the original gmail account.
Explanation
I'm using Thunderbird 14 on a portable installation with Google as provider, Lightning and Mail Merge. I have set up a Thunderbird account with different identities. I have another account (communty@gi–etc.com) which forwards to my Gmail account (gjlewis37@gmail.com). I have set up an identity on my Thunderbird account which has the email and reply to account set to community@gi–etc.com.
So I select community@gi–etc.com in the 'from field' when sending emails. However, any emails I send like this from community@gi–etc.com appear to recipients as sent from gjlewis37@gmail.com, although it keeps the identity specific signature but not the 'sent from' field.
The precise set up for my Gmail alias community@gietc.
The problem remains the same if I leave the reply-to field blank:
Yet when I send emails like this, although the signature remains, the email is sent 'from' my Gmail, and not the identity:
Best Answer
Transfer (unregistered) OP's old solution to answer.
Gmail appears to ignore all aliases not set via Gmails own 'add another email address you own' function under 'accounts', so all outgoing traffic from Thunderbird under an identity is funneled through the original Gmail account.
Adding the address through Gmail (settings -->accounts and import --> add another email address you own then follow instructions) solved the problem: emails sent in Thunderbird under the identity appear to recipients as sent from community@gi--etc, and not gjlewis37@gmail.com