Mail version 4.2 Bug

The native email program for Mac, "Mail" version 4.2, seems to have a bug.
Whenever I click on a web page link within an email, two web pages open instead of just the web page for the link. The first web page is my "home page" on my default browser (in my case, Firefox 3.6.3), and the second page that opens up is the link within the email.

I tend to think that this is a bug with the Mac Mail program, as opposed to a problem with Firefox.

Is there a fix for this bug?

Thanks for any tips.