Prior to Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), prompts (that is, alerts and other modal prompts) were window modal. That is, when an alert occurred, it blocked the user interface on all tabs in the window until the user dismissed the prompt. Gecko 2.0 introduces tab modal prompts, which only block the tab with which they're associated. Only the window.alert() method gets this by
