Today I discovered a really nice FireFox extension by a fluke.
It is called FireNES and adds the ability to play old NES games in your FireFox browser. You can choose games out of a database containing over 2500 original NES game title. And the best thing is, you do not have to download any roms. It loads the games from an online database.
After installing the extension you will see a new sidebar (if not klick on “View -> Sidebar -> FireNES” to activate it). There you can choose the game you want to play.
The extension needs Java-Script to run. If you use NoScript or any other script blocker, you have to allow scripts for the original website where you get the extension from.
Sadly the extension needs the Sun Java browser plug-in to run. It will not work with any free Java browser plug-ins.
Download FireNES: http://firenes.com.ar
The website is in Spanish language. But it should not be a problem to find the dowload link :-)