Assuming you want to use Firefox's right-click menu on the site, bypassing the site's custom menu, hold the Shift key while right-clicking. It might also be possible to inject a script into the page that modifies how a plain right-click works, but that would take some analysis to figure out and program.

If that isn't what you meant, could you explain in more detail?

Thanks for your response. Sorry, actually the opposite.

I have the context menu (r-click menu) always-on, I have a problem it being disabled/overriden by websites being oftentimes deceptive.

However, I'd like to use Onshape (browser based CAD, at https://cad.onshape.com/ . I'm trying to migrate from fusion.)

And Onshape requires mouse gestures & context menus off for effective usage.

But now that I hear there's a shift+r-click option maybe I'll reverse things and try to get used to that method...