Open YouTube videos by default in Theater Mode
Screen space is valuable and YouTube theater mode really helps, in case you’d like to avoid full screen and do more while you watch your video. Unfortunately YouTube doesn’t include an option of making theater mode default.
The problem is solved easily in the usual way: simple extensions for Chrome, Firefox, Opera and Microsoft Edge can make theater mode default. Note that you can always leave theater mode and watch a video in a smaller pane by clicking the little square icon at the right bottom, just like you did before to activate theater mode.
Extensions for Chrome, Firefox and Opera will do just this, they will make theater mode default immediately when they are installed, you don’t have to configure anything. If you change your mind later, just uninstall them. The extension for Edge is richer, providing more options you can use or not at will.