Firefox has not enabled by default "tracemonkey" yet. If you get one of the beta or nightly builds it should be turned on.
But Safari doesn't have its own new, fast JS interpreter (Squirrelfish) yet either - I'm only using v3.2.1 (http://en.wikipedia.org/wiki/Safari_(web_browser)#Mac)
Does this mean Firefox is slow because of its Canvas implementation, and not the Javascript?