This has happened to me also when i installed Firefox. What i had to do was set Firefox as the default browser to open up the links in the e-mails. Before it was IE.
DO THIS:
In Windows XP: Go to Add and Remove Programs, Set Program Access and Defaults. In “Choose a default web browser”, check on the “Internet Explorer” option and click the OK button.

In Vista/7: Access the Default Programs applet from the Start menu or Control panel. Click on the “Set program access and computer defaults” (last option) then expand Custom. In “Choose a default web browser”, check on the “Internet Explorer” option and click the OK button.