It's not an add-on; it's a default, difficult-to-remove-entirely integration into the browser. Surely that must have taken more engineering work than to simply ship it as a default add-on, which is why people are questioning the motives behind it. How about instead of installing extra, third-party services into the browser that people may or may not want, Mozilla improves the add-on search experience to direct users to add-ons they may like? Or the first boot experience?
And I think comparing Firefox to Chrome is just legitimizing the argument; Firefox markets itself essentially as the opposite of most of the things Google does. "It's okay because Chrome does it" doesn't make me feel very good about Firefox.
It's a feature that folks had been asking for in Firefox and was originally planned a while back. The resources to do the actual service side were better used elsewhere, though.
I only referenced Chrome because the post I responded to specifically mentioned Chrome for comparison.
And I think comparing Firefox to Chrome is just legitimizing the argument; Firefox markets itself essentially as the opposite of most of the things Google does. "It's okay because Chrome does it" doesn't make me feel very good about Firefox.