Moving back from Firefox 3.5.x to 3.0.13 was probably one of the smartest things I've done in a while, but that's not to say 3.0.13 is perfect either. For some reason whenever my master password is required at the moment the above dialog box will pop up followed by the actual authentication dialog box. Not quite sure what it is, when I typed "grilled cheese sandwich" or "Senjougahara is insane" nothing happened, so I'm assuming this means its a dummy of some description.
Microsoft were always fabled for having the most cryptic error messages, but what's more cryptic than a dialog box with nothing whatsoever? Elwood Blues would probably dig it.