Please come to your question and remove your email addresses as this is a public forum where spammers and bots will find them.

No idea of what account your talking about as there is many. You do not need a account to run Firefox. There are Blogs and mailing lists and Sync and......

If you are talking about a Firefox Account which is used for Sync and other services, start with this support article" https://support.mozilla.org/en-US/kb/add-secondary-email-address-firefox-accounts But you will need to verify the addition of a secondary email address thru the primary email address. If you no longer have access to that comcast.net email account you're screwed.

Then you can make the secondary email address your primary email address. https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts