As it's a simulator, not much can be said about the performance of Firefox OS, but at least you can take a complete spin through its interface and even try some applications like Maps, Calculator, Calendar, and Notes and even edit images on your computer.
Most of the functions, like the browser or the contact list, can be tried with no problems, so it's possible to get quite a good idea of how this operating system will be.
Whether you want to program applications for this system or you're just curious to see what Mozilla is preparing, this extension allows you to get an excellent idea.
A Firefox extension. This is the file URL: https://ftp.mozilla.org/pub/mozilla.org/labs/r2d2b2g/r2d2b2g-windows.xpi