In fact I've never played Nintendo, I started with "TV Game" and "Atari" and then "Sega" console but the games must be the same in those times however you can play NES games in FireFox by these simple steps :
1- go to FireNess website and hang around for a while may be to say thanks to plug-in developers
2-download and install this file
and remember it needs java to work, the other problem that site mentioned is that the keys are not working which in 95% of times the cause is that the page does not have focus on it.
you can access NES plug-in in tools menu, Have Fun !
Source: Ask the Admin
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