Good point. I'm pretty sure this component of resistFingerprinting is derived from Project Fusion, which uplifts privacy/anonymity-related changes from Tor Browser into Firefox. In the Tor Browser threat model, the idea is that you can't avoid looking like a Tor user, so the goal is to make all Tor users indistinguishable from each other. Flipping this pref as a regular Firefox user is incompatible with its primary intent/threat model, so it fails to deliver and may even make you _more_ identifiable in some circumstances.

This is a great example of why I'm generally skeptical of these scattershot approaches to making users more secure by changing default settings in mainstream browsers. Security and privacy features always entail tradeoffs and should be designed and implemented holistically for best results.