Until Firefox leadership decide to make the same change "because that's what Chrome does". Sadly, over the history of Firefox (and before that, Mozilla/Seamonkey) the leadership there has always been WAY too obsessed with following IE and/or Chrome rather than just building the best browser and taking some chances.
Seriously, trawl through Bugzilla sometime and look how many bugs are closed with the the justification being some variation of "That's how IE does it" or "IE doesn't support that", etc. And then substitute "Chrome" for "IE" later in history once Chrome took over the universe.