Comment 3 for bug 189195

Actually, on Firefox 2, the middle-click behaviour was more sensible - with middlemouse.paste set to 'true', middle-clicking in text boxes would paste text, but not give this annoying (and modal) message. Middle-click to paste is useful across X-based apps, so disabling it is unwise, but with Firefox moving away from modal boxes altogether, this "URL is not valid" box shouldn't appear at all.