This addon is apparently (as far as I could find) the only one of its kind that lets you choose a delay between each page load. (Which, btw, is an absolutely *ESSENTIAL* feature for a multi-url opening addon! No idea what the other developers are thinking).

UNfortunately, this addon here, by default, opens itself in a tab at browser start. This, theoretically, can be disabled in its options -- if only the setting would be stored permanently. No. For some reason if you change the setting, this is also "forgotten" upon next browser start, i.e. it's always on "default", making it extremely annoying. I have to resort to disabling and then re-enabling the addon every time I need it, hence the -2 stars.