If Mozilla wanted to promote choice, why not present the user with an option of some sort? I fail to see how picking s default provider counts as "choice".
But the choice isn't by default. Users must go explicitly choose. If Mozilla wanted to "promote choice", they'd prompt the user to pick a search engine on first use. Or use a default and make noise about changing. Like how FF reminds you about your rights, or asks to join customer feedback program.
I don't see how Yahoo promotes choice over Google, unless by choice you mean promote competition by going against the market leader. Which is fine, and I dislike Google, so good on y'all. But I don't get the point if euphemisms.