Since Firefox 4.0, some alert messages may popup from the main window instead of in the message dialog. Although the alert message can handled in the other window, it may be confusing why the browser isn’​t responding. To revert the alert message behaviour to separate windows, the Firefox configuration needs to be altered. To do this, open a new tab or window in Firefox and type about:config in the url bar. Filter on prompts to find the correct option. Change the value of prompts.tab_model_enabled from true to false. Alert popups will now be separate windows again.