When Flash is interacted with (like you clicked the play button), it becomes the target for all keyboard input until it loses focus. So if you watch a video and draw a gesture which sends keys like CTRL+W (to close the window), it won't be acknowledged by Firefox because the video player is receiving the keyboard input. You have to click somewhere outside of the video player first, then keystroke based gestures should work fine.
This isn't a bug with any gesture app, nor Flash, nor Firefox; it's functioning as intended. Remember that a Flash game (for example) may very well have hotkeys like CTRL+W which is supposed to do something, so when you interact with Flash, it gets all input until it loses focus.