I don't know how greasemonkey works on firefox, but with tampermonkey on chrome you can give userscripts whitelists of sites they're allowed to run on.
I guess tampermonkey reads this comment to set the site:
// @include    http://forum.blockland.us/index.php?topic=*
The youtube embed and ignore scripts all run on these forums exclusively for me, although I've uninstalled the ignore list because I'm having issues with it.