i know this is an old question, but it's exactly the one i've been trying to find an answer for, and the ones here don't seem to fully satisfy the original question.
It appears to me the only way to regulate just context menu access to specific sites is using Firefox's "hidden" CAPS feature:
https://www.mozilla.org/projects/security/components/ConfigPolicy.html
It can be tricky to manage the policies, and any time you want to add a new site to the list, you have to edit the user.js file & restart Firefox.
i'm not really a fan of using add-ons to manage built-in features, since i want the policies to be persistent & i see no reason to let Firefox suck even more memory for an add-on i'll rarely utilize once my settings are established.
The only question i have about doing this through CAPS is how to address the actual context menu option.
I would really like to know this as well! – Cerberus – 2012-02-15T08:27:50.940
1
I have found a way, through a Greasemonkey script! See here: http://superuser.com/questions/366016/firefox-right-click-easy-support-for-the-firefox-or-the-sites-menu/443734#443734
– Cerberus – 2012-07-01T22:16:02.120