There is normally no need to disable this setting as all you need to do is to hold down the Shift key when right-clicking to get the Firefox context menu.

It's just an example. I'm looking for an answer to my actual question please.

This pref only has a global setting that is applied to all websites,. You may have guessed from my above reply that it is not possible to make such a pref work on a origin base like exceptions permit. You would have to use a second profile with the other setting for this pref and use that profile where you want to see the website context menu.

If I wanted guessing games, I would have played Jeopardy.

Next time you might try a more unambiguous "I don't know a way to do that.", Riddler.