If you are looking to overlay video on top of what you are doing, I do not think that us a feature like it used to be.
ATI used to have a feature called ThruView that allowed just this.
Now your best bet would be to look for 3rd party software to make your browser window transparent with alpha blending or similar. Then you could have the video in the back on one browser and use another for surfing.
EDIT
It is much more easy check out this(post 6).
Make a window semi-transparent? @ http://autohotkey.com
Because AutoHotKey can be used to macro in games, it is recommended to have it OFF when playing online games(just in case).
EDIT EDIT
This may have a performance hit(shame). Tested with Media behind Firefox