This makes me less likely to use Mozilla, but I was already a Chrome or Safari user (despite supporting the Mozilla mission). Seems like a bad decision for Mozilla.
Why does this make you less likely to use Firefox? The default search engine is just that: the default. You can change it to something else, and always have been able to. If you care enough to know what search engine you use, you should be able to change it.
You can't change the default search engine in Firefox for Android, at least through the UI. And mobile is a high-frequency user of search, which makes that decision even more interesting.