As much as I’d like to help you, even though I think it’s not a good idea (one can still open the dev tool using the regular menu), I’m not sure exactly what changes needs to be done where to implement what you want. Stuffs like disabling F12 and right click, would probably need to be done in one of the script that is injected in every page, like this one. All the visible components are actually part of the styleguide, which the browser extension uses, so I would look there for front-end shenanigans.
There was talk about it, I think we should create a feature request for this. Feel free to create one. Ref. Firefox keyboard shortcut missing