Is the Comcast account POP or IMAP, and if it's POP, are messages still on the mail server? If the gmail is IMAP, it would be possible to upload mail from the computer in limited numbers at a time. But if the Comcast mail is on the server, the easiest way to transfer to gmail would be to have the gmail account collect mail from the Comcast account, through gmail Settings, Accounts & Import.

Both Comcast and gmail are IMAP. The Comcast server does not have the email I need moved. The "Saved" folder is a Comcast.net folder in Thunderbird. Is there a way to move or copy the entire "Saved" folder, including all of its subfolders and email to a "Saved" folder under gmail.com? How does the Thunderbird profile identify which folders are associated with each email account? Can the profile be edited to change the account association for the single folder "Saved" and thereby transfer the entire folder plus its subfolders and email to a different account?

If the Saved folder isn't on the mail server, but stored in a local folder, you can transfer messages to a gmail IMAP folder by drag and drop in TB, in limited numbers as explained above. A folder in Local Folders isn't associated with any account.