FireNes is a little Firefox widget that allows you to instantaneously play any NES game in your browser. It’s free and it’s incredibly easy to install and use and it’s probably going to ruin my life because I can’t stop playing with it. It requires only a one-time, 5-second download to have a sidebar chock full o’ NES goodness.
The FireNes site is in Spanish; here is a not-too-terrific English translation. All you need to know is this: go to FireNes, click on “Install” (“Instalar” if you’re on the Spanish page and are feeling fancy), then relaunch Firefox. Go to the Tools menu and click on FireNes. It will pull up a menu of any game. Right click on the game to edit options and learn which keys to use.
You’re lucky I even wrote this post considering I’ve spent the last 3 hours “researching” the subject matter. And by “researching” of course I mean re-mastering Q-Bert and amassing coins in Mario-land- because they have a bunch of retro games on there in addition to the NES favorites.