Firefox has a constellation of bugs where it behaves differently depending on whether or not a <p> is explicitly terminated, inside a <div> which is enclosed in a link. The <p> need not be the final object in the <div>. The bug is present in Firefox 18.104.22.168 and 3.0.1 (tested on a Mac); it’s still there in Firefox 3.5.7. Safari on Mac and IE6, IE7, & IE8 on Windows behave as one would expect. Here is a self-contained demonstration (opens in a new window). Filed as Firefox bug #436600.