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Firefox » Javascript alert box style

How to change javascript alert box back to window instead of the new style with overlay

New style looks nicer, but if you need to access some web-based systems with remote desktop on slow mobile connection like me, this is very slow to work with (overlay happens to transfer a lot of graphics data from remote desktop). In such case, it is better for me to have old-style window javascript alerts.

Basically, instead of this...

...having this back:

Solution is easy: Type about:config and change the value of prompts.tab_modal.enabled to false.