With the removal of custom toolbars via legacy add-ons, users are left with no way to develop/enable full-width custom browser UI. The sidebar API provides great flexibility for customization, but being a side panel, it is not suited for content that requires a lot of horizontal space. This is particularly problematic for low-vision users who use large fonts. Perhaps the sidebar could become dockable to the bottom of the screen? A more responsive layout would open up options for using the existing Web Extensions API that catered to different user needs without adding much/any API surface area.

A full width browser UI sounds an awful like a tab or window. But this has been requested in other bugs like the toolbar API bug 1215064 and bug 1373504. Will dupe to the toolbar since that's got the best chance of happening.