This is my Firefox browser maximized - via Alt+Space, Ma
Ximize
This is my Firefox browser full screen - via F11
No title bar, no menu bar, no URL bar, no tab bar, no add-on bar etc - just the viewing area, I only use it when viewing a video.
Why would anyone want the equivalent in Sigil, i.e. no title bar, menu, toolbars, etc. Even I don't do everything with keyboard shortcuts… too many to remember