One of the most annoying habits Firefox has is stealing the focus from other programs you are working in. Clicking on a hyperlink inside programs such as email or office type applications causes the link you clicked on to automatically open inside Firefox. For example, I'm reading email, and I click on a link inside the email. All of a sudden I'm looking at a Firefox window showing the web page that the email link goes to.

via How To Stop Firefox From Stealing Window Focus.