OK, you posted here with Firefox 3.6.15.
What does your email address have to do with using Firefox?
What do you mean by "Firefox insists on dialing my old one"?

I mean that when I restart my computer Firefox dials my old email provider, earthlink. How do I get it to access my Gmail address?

Firefox doesn't do email and it doesn't "dial" - the operating systen makes the connection to the internet for both dial-up and for always-on broadband service.

If your problem is that when you click on a "mailto" hyperlink on a web page, that Firefox loads Earthlink webmail and you want it to load Gmail - see this.
http://support.mozilla.com/en-US/kb/Changing+the+e-mail+program+used+by+Firefox

Sorry to use technical terms that you may not understand, but I had to guess what you meant.