Firefox lets you set 'DNT: 1' if you don't want to be tracked, using the setting you describe, but does not have a setting to send 'DNT: 0', indicating that you

Firefox lets you set 'DNT: 1' if you don't want to be tracked, using the setting you describe, but does not have a setting to send 'DNT: 0', indicating that you are okay being tracked. As a result, there is ambiguity about whether lack of a header means nothing or means "tracking is ok". If browsers let you broadcast your willingness to be tracked, advertisers wouldn't be able to make the (bad faith) argument that it's ok to track just because someone didn't set the header.