When moving the cursor out of the tiny window pane given to you by most Flash games, unpleasant things tend to happen. I hate that, so I prefer to play fullscreen. But for most Flash games, to do that you need to find hacked versions from non-original sites, and they are often old versions.
To make any Flash game play full screen from their original pages, even when it doesn't have an option to do so, I'm using the Flash Game Maximizer add-on. You just click an icon in the lower right corner to maximize the flash game in the browser window, then hit F11 to use Firefox's fullscreen view.
Upon doing that, you'll find that Firefox's fullscreen view still has a popup bar at the top of the screen that you can't get rid of, which sucks. To get rid of that, I use Full Fullscreen.
To make any Flash game play full screen from their original pages, even when it doesn't have an option to do so, I'm using the Flash Game Maximizer add-on. You just click an icon in the lower right corner to maximize the flash game in the browser window, then hit F11 to use Firefox's fullscreen view.
Upon doing that, you'll find that Firefox's fullscreen view still has a popup bar at the top of the screen that you can't get rid of, which sucks. To get rid of that, I use Full Fullscreen.