The single largest advantage of Firefox over other browsers is that despite all odds and occasional missteps they managed to respect users' desire for complete privacy.

  For Firefox we want to better understand how people use our 
  product to improve their experience. 

Sure thing. But the fact that they are unhappy that some (many?) people are opting-out from the data collection is merely a sign that they don't want to understand why people are using Firefox in the first place. By opting out from the data collection people effectively tell them over and over again that they don't want for Mozilla "to understand how they use Firefox" or "to improve their experience", not at the expense of their privacy.

No phoning home. No telemetry, no data collection. No "light" version of the same, no "privacy-respecting" what-have-you. No means No. Nada. Zilch. Try and shovel any of that down people's throats and the idea of Firefox as a user's browser will die.