What is your account type - POP or IMAP?

POP... Sorry

With POP you only get access to Inbox on the server.

If your question is about getting messages in Sent on the server into your local Thunderbird, you'll need to set-up your account for IMAP.

You can place a copy of every message sent into the local Sent folder though.

Thanks... Am I able to setup Comcast as IMAP?

Follow these instructions. http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

If the new account wizard defaults to POP, use Manual Setup, and enter the settings for IMAP manually. https://support.mozilla.org/en-US/kb/manual-account-configuration

Make sure to get the correct IMAP settings from Comcast beforehand.

Talk to Comcast in your area and make sure they offer IMAP. They are just now allowing IMAP instead of POP but I am not sure how wide spread this offering is.

This page has and IMAP application forum. https://xcsignup.comcast.net/onboardingapps/imap