After the bug 1208596 is fixed, WE addons can have its custom sidebar panel. However, some type addons like Tree Style Tab or Tab Center will hog the sidebar area while Firefox is running. For example, it will be stressful that I have to switch the sidebar to "Bookmarks" and switch back to "Tree of Tabs" at every time. Thus something mechanism to provide second/third sidebar panel area is required. I have two plans: A) Providing ability to embed "sidebar" widget at inside of any sidebar panel, for WE addons. (= new WE API) B) Providing second/third sidebar panel by Firefox itself. How about this idea?
I can imagine doing something [different than bug 1208596] to support tab managers separately from sidebars, but I don't see allowing multiple sidebars as a feature we'd implement without Firefox product and UX driving that decision and design. We should make sure there is a bug for tab management and look at doing an experiment.
look at after the minimal patch landed - look at UX for sidebar extension use cases