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:
define('FORCE_SSL_ADMIN', true);
define('FORCE_SSL_LOGIN', true);
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.
Solution:
Log out from wordpress
Comment in config.php:
//define('FORCE_SSL_ADMIN', true);
//define('FORCE_SSL_LOGIN', true);
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.