FlashResizer extension attaches a small vertical line next to flash objects. You can drag and resize video or game window by clicking the line. By double-clicking that line, the flash player will automatically adjust to full screen mode.
From the FlashResizer option you can adjust size and color of the vertical bar, add websites that requires these settings and you can enable full window mode.
You can install FlashResizer at Mozilla add-on site. (via – techtrickz)