Unfortunately by sending DNT you are merely suggesting to the server that you wish to not be tracked. There’s no requirement for the server to actually care about you at all.
Now, if DNT were actually legally binding though - that would indeed be very cool.
There isn’t a built-in option for that, but you can hack it with custom styling rules using userChrome.css. Just like the other guy said though, the styling rules that work now may need to be updated if something changes in Firefox, but that is the nature of customization.
Here is a link to a stylesheet that achieves this.