Check out "Personal Titlebar" Rated 3 out of 5 stars
I did try this "Hide caption Titlebar" but then I foiund that this is really not doing what I wanted. I wanted to move the "menu bar" up to the title bar. This add-on does that but you have move your mouse to the icon on top left to make that activated for a few millisec.
I found "Personal Titlebar" at https://addons.mozilla.org/en-US/firefox/addon/personal-titlebar/
that instead just moves the menu bar to the title bar and keeps it there.