A Firefox addon that appends nonsense tracking parameters (?utm, clickid, etc) to most requests, just to poison the data collected by trackers.
Turns out this thing already exists, at least for Chrome:
https://hello-kill.github.io/
That project might either be a joke or simply dead. The corresponding github repo has no plugin, and there's nothing on the chrome store (obviously).
The concept is sound, though; it's trivially easy to fake tracking data, and only a little bit harder to build a tracking token sharing platform (to deliberately screw with analysis that links tracking tokens to location or other data).
