If you see this error in firebug: Access to restricted URI denied (NS_ERROR_DOM_BAD_URI)…
This is likely you have set in your WordPress installation:
This is a severe bug in Simple Forum.
In this case, Simple Forum will not simply install, actually, it’s AJAX components will not like https protocol prefix.
Log out from wordpress
Comment in config.php:
Log in (with a normal http link)
Perform installation of the Simple forum Plugin. Everything should go smoothly now.
Uncomment the commented before. Log out/log in back with https.
May be also valid for other plugins.