On Firefox, the "Tell websites I do not want to be tracked" is not enabled by default. I don't understand why this is not the default action.
The option should be "Tell websites I'm okay with being tracked" and should be ticked off by default.
Actually, it is unclear what the "do not track" actually means. Does in include "do not log" - or "wipe my IP address from the logs after x days"?
Anybody, who does want to not support the biggest trackers (facebook, google, twitter), should (a) deactivate sending cookies to third parties and (b) install ghostery in FireFox. That's much more effective than the "do not track" additional information sent to the tracking web sites.