You will likely have the mailto protocol set to open Firefox. For details of how to fix that see the "Change the action for a content type" section at Firefox repeatedly opens empty tabs or windows after you click on a link, specifically look for the mailto content type.

I was having a similar problem for as long as I have had Firefox on my mac (at least two years). It would always open many tabs whenever I clicked on and email address link in the browser. I am on an Intel Mac with updated Leopard operating system. At one point, I was able to try to close firefox and it notified me that it had ~4,900 tabs opened. Starting today, firefox freezes immediately whenever I open it. I tried removing it and reinstalling and I tried reinstalling older versions, but the problem persists.

If Firefox tries to reopen all those tabs then you need to remove the files sessionstore.js and sessionstore.bak and any existing files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js to prevent Firefox from restoring a previous session.

See http://kb.mozillazine.org/Session_Restore

Set the pref browser.sessionstore.max_resumed_crashes to 0 on the about:config page to get the about:sessionrestore page immediately with the first restart after a crash has occurred or the Task Manager was used to close Firefox.

That will allow you to deselect the tab(s) that you do not want to reopen, but will allow to reopen the other tabs.
See:

• http://kb.mozillazine.org/Session_Restore#Restoring_a_session_after_a_crash
• http://kb.mozillazine.org/Browser.sessionstore.max_resumed_crashes

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.