From Sega Retro
The Sega MegaFire control pad (or Sega Genesis MegaFire control pad in the US) control pad was the first official "turbo" variation of the Mega Drive Control Pad. It debuted in the early 1990s and is essentially an extended three-button controller.
The MegaFire is extremely similar to a stock three-button pad, however features three turbo switches placed above the , and buttons. These allow a turbo mode for each button to be switched on or off, meaning rather than a player having to repeatedly press a button for a repeated action, they can simply hold it down and let the controller do the work.
Turbo Mega Drive controllers are common, partly because the system had a large library of shoot-'em-ups in its first few years on the market. The MegaFire is not particularly notable, however, as future controllers would allow the user to adjust the speed of the turbo option, and apply these functions to six-button controllers.