It only switched your search engine if you'd never changed from the original default (Google). If you had ever switched the search box to use a different engine, FF continued to respect that.
But why would I change it if I wanted Google, which incidentally is, I'm sure, what most people want? Leaving the default can be as much a choice as changing it.
(I'm on Mozilla's side in this discussion, but that particular move was terrible, even if not too consequential, in my opinion.)
You mean besides that time Firefox reverted everyone's search engine to Yahoo?