I wrote a new browser addon to play flash games full screen. It works by switching the screen resolution automatically, so this keep the games fast even on not super powerful pc and graphic heavy games.
It's for IE, Firefox and Chrome and I called it SwiffOut. It's freeware and you can get it here : http://swiffout.com
There is actually a very very easy way to play flash games full scren. All you have to do is to view the source code a bit and walla! You're playing Sonny 2 Fullscreen
SwiffOut also switches to the most appropriate resolution, this keeps very good performances even on graphic heavy games and small PC. Selecting the closest resolution of a game manually is not a super easy task as well. It also works better with some games like 'final-ninja' which doesn't scale in browser. And you have to hide the browser, and your taskbar...
The mozilla store dropped my addon due to the fact that I asked a partial review instead of a full review. Binary addons must be fully reviewed. You can get the extension from http://swiffout.com in the meantime.