GNOME Bugzilla – Bug 437109
Javascript window.alert() blocks other windows (and tabs)
Last modified: 2016-11-26 13:37:31 UTC
When a document uses window.alert() and the user leaves that alert window open, trying to navigate using other windows stall until the alert window is closed. Simple to reproduce. Open up two windows. Focus first one, and enter javascript:alert('test'); in the location bar. Leave the alert box open. Switch to the second window, try to navigate to any website. I just tried this with Firefox, it worked fine.
Indeed, those dialogs are modal, even with 3.6, and they block interaction with the rest of the app. Not sure how relevant it is these days though. Maybe the window/tab in which the alert originates from could grab the focus.
*** This bug has been marked as a duplicate of bug 516815 ***