Just discovered a setting in Firefox, not really sure how long it has been there, but I've only just noticed it trying to change a setting.
Thanks to google I found this:
http://support.mozilla.com/en-US/kb/how-do-i-stop-websites-tracking-meAt first, I thought great, then...
When you turn on the Do-not-track feature, Firefox tells every website you visit (as well as their advertisers and other content providers) that you don't want your browsing behavior tracked. Honoring this setting is voluntary — individual websites are not required to respect it. Websites that do honor this setting should automatically stop tracking your behavior without any further action from you.
So it may not be as useful as I first thought...though something to have checked, for those that will honor it.
Anyone know when it was introduced?