Perhaps I'm spoiled, but I'm wondering why this wasn't a thing many years ago. Am I wrong in saying this was brought about in reaction to Chrome adding the same feature, or has this been there for a while? Clearly people wanted this for ages, hence the work on things like PhantomJS. But it seems like it was only added once they realized they needed to play catch-up with Chrome.
https://bugzilla.mozilla.org/show_bug.cgi?id=1338004
So not in reaction to Chrome.
In fact, the first attempt for this was almost a decade ago (https://bugzilla.mozilla.org/show_bug.cgi?id=446591)