You can make firefox fullscreen _within the window_ by changing a flag in about:config ... full-screen-api.ignore-widgets (set to true)
then, when you press `F` on a video, you will remove all firefox 'decoration', just like fullscreen mode, but it'll respect whatever position and size you have set for the browser.
As an additional tip, if you want to get rid of the "You are now fullscreen" (or whatever it says) that comes across the top... you can set the following to 0
then, when you press `F` on a video, you will remove all firefox 'decoration', just like fullscreen mode, but it'll respect whatever position and size you have set for the browser.