Looks like I found a rather silly bug that appears in Firefox 4 for Mac. It’s already known and up on Bugzilla as bug #632606. This is what happens:

1. Start Firefox 4 on Mac OSX 10.6.6
2. Drag primary Firefox window onto a MDP-connected secondary monitor
3. Press Command-N. What should happen is Firefox creates a new web browser window. But in this state Firefox just ignores the command.
4. If you drag the primary Firefox window back to the primary monitor, Command-N works just fine and dandy.
5. If you drag it back and forth you can get this bug to show up or not.

So if anyone is out there and pounding away on Command-N or expecting your Download Manager window to appear, you might want to check to see if Firefox is on your primary or secondary display monitor.

Silly little bug, really. 🙂