Revert "Remove use of Components.utils.methodjit"

This reverts commit 53a4d987b60fafcb253de16a0cf5073a4406a719.
Only use Components.utils.methodjit when supported

As Boris Zbarsky pointed out in
https://bugzilla.mozilla.org/show_bug.cgi?id=878679,
the setTimeout() hack doesn't help in Firefox 23 and later. For now we
use the useMethodjit hack when available, so that performance won't
regress for Firefox 23, and cross our fingers that
https://bugzilla.mozilla.org/show_bug.cgi?id=776798 is fixed for
Firefox 24.