This is what my user.js file looks like:
user_pref("capability.policy.policynames", "allowclipboard");
user_pref("capability.policy.allowclipboard.sites", "http://www.forums.wincustomize.com");
user_pref("capability.policy.allowclipboard.Clipboard.cutcopy", "allAccess");
user_pref("capability.policy.allowclipboard.Clipboard.paste", "allAccess");
I also have javascript enabled.
FF3 right click copy and paste is working here for me on these forums.