Any window (for any program) can stay on top of others with a keyboard toggle, allowing you to browse while you watch. Basically picture in picture (though not for full-screen games), that you can move about/scale/minimise etc.
You can use autohotkey to use this script:
^SPACE:: Winset, Alwaysontop, , A
Once auto-hotkey is installed, create a new plain text file, copy and paste in the above and save it as a .ahk file. Run it (double click) and it will work on any currently active window by pressing ctrl+space. I set my hotkey to F5.
Or
You can download a program that does the same thing, but you can't change the hotkey from ctrl+space.
With Twitch you can pop videos out into a new window that's already stretched by clicking the cog wheel and selecting Popout Player.
To stretch youtube videos to fit the screen and hide the rest of the stuff, change the 'watch?v=' part of the url to 'embed/'. Or you can use this firefox addon.
Or ImprovedTube extension for chrome - second item on drop down, player size - fit to window edge. Toggle back to normal view by clicking the theatre button.
Use the firefox Hide Navigation Bar addon to keep streaming video windows cleaner with a toggle.
Toggle the bookmark bar with ctrl+shift+b in chrome.
And to automatically launch the script at startup, copy the ahk or exe (whatever you went with) file, press win+r and type:
shell:startup
Hit enter, right click inside the folder and click paste as shortcut.
Smooth auto-reload for new comments:
-I also recommend using this Live Thread extension (firefox and chrome) that updates neogaf threads automatically at a customisable interval, triggered with a refresh button on the page near the top-right of a thread or at the bottom near the page select. It works well with the keep window-on-top script above for when you want to be lazy or are doing something else.
It remembers your preferences for threads and when it reaches the bottom of the page, it auto-loads the next comments below the last one of the page under a page break. I've been using it for a few days now, it's really good. Especially useful for big events.