I can't say that I have used the Firefox OS simulator extensively but I have been fiddling around with it a little bit and a lot of stuff didn't seem to work for me at least (like landscape mode, buttons in the notification menu, scroll seem to behave very weird in lists and/or gets stuck quite often resulting in me accidentally clicking on stuff, strange text formatting so that text gets printed outside of the screen, it crashes when I turn on WIFI, can't seem to change screen resolution, etc.). So "the best" might be pushing it, especially compared to the Windows Phone emulator and tools which are really, really great (but sadly won't be used by most mobile developers for obvious reasons :) ).

However I am excited to see where Firefox OS might be heading even though I think they will have a though time fighting it out with other potential "third spot" platforms for starters.