I don't understand. If you want an integrated suite, use SeaMonkey. If you prefer individual stand-alone programs, use whatever browser and email client that you want. There is no special relationship or dependency between Thunderbird and Firefox that obliges you to use both.

If you do choose to use Thunderbird and Firefox, each can be set as the relevant default program, so that links for email you see in your browser would open Thunderbird, and URLs in your email would open in Firefox.

I set thunderbird and Firefox as defaults respectively . When thunderbird is opened there is no mail link in the browser except to comcast mail

Do you mean that when you click an email address link in a web page, or use the Email Link feature, you want Firefox to automatically send that to Thunderbird? Please see this article: Change the program used to open email links.

Is that what isn't working?

Or if you mean that you would like to add a Mail button on the toolbar to launch Thunderbird (without necessarily composing a new message), please see this thread for suggested add-ons to create such a button: https://support.mozilla.org/questions/1133489