Not technically full screen because of the window border and title bar, but close enough for web videos: Under Firefox, install the PopVideo extension. Allows you to pop out the video into its own window. Now you can maximize the video window to a single monitor, even in Eyefinity/Surround mode.
BTW, playing local video files presents the same problem of full screen videos spanning all three screens. The fix for that is MPC-HC. Play video, maximize the window, then press "1" to remove the UI. Press "3" to return the UI. Space for pause/unpause.