The #DoNotTrack header has been deprecated, and has been largely useless because it was never adopted by most advertisers, using the cop-out of it not being legally binding. Despite this, #Firefox (and most other browsers, with the only exception of Apple's Safari AFAIK) still support sending the header, despite it being arguably a waste of bandwidth and implementation resources (UI options to control its settings, JS access to it, etc). Why do they still do it?