Not all multitasking need be productive. If you like to watch YouTube clips while you are otherwise engaged in productive pursuits online, then you have likely juggled two windows, each resized just right so that you can conduct your business in one window while in the other your YouTube clip plays. If you don't mind clips playing in a very small video player, give Firefox add-on Side Watch a try. It lets you play YouTube clips in Firefox's sidebar so that you can keep a casual eye on a video without sacrificing much screen real estate.
When you install Side Watch, it places a button in Firefox's add-on bar at the bottom of the window. With Side Watch installed and your add-on bar visible (View > Toolbars > Add-on Bar), head to YouTube.com and play a video. With the video playing (or paused), click the little YouTube button that Side Watch installed on the add-on bar and Firefox's sidebar will open with the video.
A couple of things: The video you started playing on YouTube before opening the sidebar continues to play, so you'll need to pause the video or close that tab when the same video is playing in the sidebar. But Side Watch does not play preroll YouTube ads, making it a great way to jump right into videos, particular those preceded with odious 30-second spots.
The video player that Side Watch places in the sidebar is the same you'd find for an embedded YouTube clip. It features basic controls, and it plays only one video at a time. That is, you cannot play playlists. Also, Side Watch works only when you are on YouTube.com; it can't move embedded YouTube clips to the sidebar.
You can resize the sidebar, but the vertical size of the player doesn't change, which in effect means you cannot adjust the size of the video player. There is, however, a full-screen button you can use, but that gives you really an all-or-nothing option in terms of resizing the video player.