It's not "at all", I just found one issue, it's a purely Firefox related problem of Content Security Policy, there's no problem on Chrome. I tried on the home page of DuckDuckGo and there's no problem.
With Firefox way of loading content scripts, you might need to re-open or refresh tabs after installing.

Your problem is because your script cancel all events happening with the right click (context menu), I can not do anything with it. In simple words, that script prevent events that could allow to get a targetted element to occur, so, there's no much way to do another way, except you activate your script when you need.