Another new feature (available in nightly builds from today): a XP SP2-like yellow bar appears at the top of your Firefox window's viewport notifying you of blocked popups. Click on it and it gives you similar options on whether to allow the popup(s).

Here's how it looks in Firefox:

Screenshot of Firefox blocked popup UI


And this is how it looks like in IE with XP SP2 (Release Candidate 2) installed:

Screenshot of XP SP2 blocked popup UI in IE


Screenshot of XP SP2 blocked popup UI in IE with context menu