You can also disable Javascript entirely except for specific sites by using policies, for example:
user_pref("capability.policy.default.javascript.enabled", "noAccess");
user_pref("capability.policy.policynames", "jsok");
user_pref("capability.policy.jsok.javascript.enabled", "allAccess");
user_pref("capability.policy.jsok.sites", "
https://www.goodsite.com about:");
Will only allow Javascript to execute on site
http://www.goodsite.com and any about: pages (like about:plugins).
Conversely you can disable Javascript for specific sites:
user_pref("capability.policy.default.javascript.enabled", "allAccess");
user_pref("capability.policy.policynames", "nojs");
user_pref("capability.policy.nojs.javascript.enabled", "noAccess");
user_pref("capability.policy.nojs.sites", "
http://www.badsite.com http://stupidpopupsite.com");
In both cases you can list as many sites as you want in the site list.