[PSA] Firefox has this built in already: Hold Shift while right-clicking. Been a while since I tested on a fresh profile, but I don’t believe this requires any preference tweaking or about:config stuff.
There's a about:config setting called dom.event.contextmenu.enabled defaulting to true, when you change to false you don't need to hold shift, it will always open the browser context menu (and in most cases still do the page behavior of showing the web app context menu or whatever, you can then press esc to close the browser's menu but it may also close the page menu depending on how they implemented it).
Then there's a second option called dom.event.contextmenu.shift_suppresses_event which defaults to true and does what you said: Simple right click opens the page menu and shift + right click don't send the page the event at all and instead opens the browser's menu.
Then there's a second option called dom.event.contextmenu.shift_suppresses_event which defaults to true and does what you said: Simple right click opens the page menu and shift + right click don't send the page the event at all and instead opens the browser's menu.