A new Firefox extension aptly named "Flash Game Maximizer" is a must-have for any Flash game enthusiast. It does one thing, and does it well: letting you toggle between standard- and full-screen modes on any game--regardless of whether it has been coded with such an option.
Considering that Flash games are hosted all over the place, and rarely with a ubiquitous user interface, you're likely to run into a good number of them that must be played in whatever space they've been given on a Web page. With Flash Game Maximizer, however, you don't have to worry about this at all.
Once the extension has been installed, you'll get a new button on the bottom-right corner of your browser that turns yellow when it detects Flash. Clicking it resizes the game to fit your browser window. A slight caveat here is that it completely restarts whatever Flash module is on the page, which on most games means losing any progress.
Because Flash Game Maximizer is an "experimental" extension, you will need to log in to Mozilla's add-on site to download it.