Firefox 64 introduces an entirely new API, browser.menus.overrideContext, which allows complete customization of the context menu shown within add-on content like sidebars, popups, etc.
This is illustrated right in the post by my favorite pain point, the Tree Stye Tab custom tab menu mixing with the regular tab menu.
I wish tab handling had a first-class API in Firefox, so that different tab managers could interact with the default tab menu, etc.