This is not a system alert, it is a Firefox alert, and it does hang out for quite a while.
Two proposed methods have been given, ALT+R [which my gut is telling me probably won't work] or using ALT+D to get to the address bar, then Shift + Tab until you hear "Alert button," which you can then activate to get focus.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back