ITT we share helpful firefox user_prefs that make it better. I'll start.
user_pref("full-screen-api.transition-duration.enter", "0 0");
user_pref("full-screen-api.transition-duration.leave", "0 0");
user_pref("full-screen-api.warning.timeout", 0);
This removes the annoying banner saying press ESC to exit fullscreen every time you fullscreen a yt video.
user_pref("media.videocontrols.picture-in-picture.video-toggle.has-used", true);
user_pref("browser.engagement.downloads-button.has-used", true);
With this in your user.js before you open the browser it won't popup instructions on obvious features. I have a pretty big user.js based on betterfox that disables telemetry and toggles all the settings the way I like them so it's completely configured on install without doing anything, and then I've added a bunch of minor quality of life conveniences like these. Share yours anons.