Yes, performances are "limited"/"maximized" to around 60 fps in browsers.
Anyway, getting 200 fps or 60fps makes no difference.
Check this other blog article by Ashley
explaining more on the subject and rendering/performances with games made in C2.
As for the information about FF15, I got it from Mozilla's dev in #games on irc.mozilla.org
There are a lot of devs working/idling in there and they teased me about that version being really awesome for games. When I asked them about hicups and small "laginess" in FF, they told me this version is supposed to correct it.
FF 15 should be released around august/september if I'm not mistaken.