This small extension for Firefox will allow you to "maximize" Flash games (and other Flash content) at the click of a button (in the status-bar and/or tool-bar, or you can hit F12 instead).
The Flash content is then reloaded in the full browser window, causing it to resize along with the window (with or without enforced aspect, or scaled to one of the predefined percentages). This allow you to play your favorite Flash games, or view other Flash content, in whatever size you like, even including the Firefox full-screen mode, where you can enjoy your Flash game with that "real-game" feeling that can only be achieved by dedicating 100% of your screen(*) to the action.
Please refer to the help screen for further usage-information (the help screen is accessible from the toolbar/statusbar-button context-menu, as well as through the Tools/Add-ons dialog).
From version 1.2.0 you get proper support for tabbed browsing, and you can even use the context menu to select between a number of different sizing modes, as well as a number of other new settings and features (some in the menu, more in the options).
From version 1.3.0 you get an option to specify the Flash quality setting to be used when maximizing. I recommend using the Medium or Low setting and enabling the "Force Flash Quality" setting, if you tend to maximize your games in full-screen, or otherwise very large, windows (unless, of course, you have a VERY powerful machine, capable of keeping a reasonably high frame-rate, even when using the High quality setting). The loss in visual quality for most games, when using Medium instead of High quality, tend to be relatively negligible, but will often result in a marked improvement of the frame-rate. Please note that after changing the quality setting, you will have to re-maximize the content, before the change will be applied to the content (AFAIK there is no way to make this "instant", like changing the size). Also, there are quite a lot of Flash content that doesn't respond to this setting at all. The reason for including this option in FGM anyway, is mainly to allow you to specify a quality setting for games that hides this option from their own context-menu.
From version 1.3.6 you get less bugs - Most importantly, the bug causing the statusbar-button to stay grayed out, if the toolbar-button wasn't also visible, has been fixed. This version supports all platforms, but only rudimentary testing have been carried out on Darwin and it remains completely untested on all the *nix platforms.
Happy Flash Game Maximizing,
(*) To achieve 100% full-screen (ie no browser GUI visible what so ever), you will need one of several other extensions offering this functionality (I might implement my own version of this in FGM in a future version, but for now...) - I recommend the "Autohide" extension (search Google, as I don't think it is available here on AMO), which can remove any or all of the remaining GUI elements (scrollbar, tool-bar etc.) from the full-screen display. If you install such an extension, and furthermore configure it to show a 100% full-screen, you will obviously not be able to click either of the buttons. In this case, you can use the various hot-key combinations, instead:
F12: Maximize/minimize Flash content.
CTRL-F12: Maximize/minimize Flash content, also switching full-screen mode on/off.
ALT-F12: Toggle full-screen mode, without reloading the content.
SHIFT-F12: Show the context-menu at the current pointer position.
Note that some/all of these keystrokes may not work if the location-bar or search-bar has the focus/cursor (usually clicking anywhere on the Flash content will fix this).