The FireNes Firefox extension integrates over 2000 old-school NES games with your favorite browser. Once you install FireNes and fire it up (either through the Tools menu or the optional toolbar button), you can browse the enormous library of games in your Firefox sidebar and play any one of your 8-bit favorites at the click of your mouse.

Xultris is a Firefox extension developed by mackers that brings Tetris addiction to your favorite browser. Once installed you start it via the Tools (?) menu with options to start a single or two player game, display a background image and a grid to help guide the tetraminoes as they fall.