Assuming you are going to use a search engine at all, they are going to collect some data and use it to sell more advertisements. That is their business model. Whether you use Firefox, Epiphany, Midori, Google Chrome, Konqueror or something else is irrelevant. What Google ends up paying Mozilla is for the privilege of being the default search engine and Google was already the default before any agreement between Mozilla and Google was ever signed and since Google is what most users use as a search engine, it is a win win situation.
The choice is plain and simple and hardly takes a couple of seconds in a very visible drop down box. Installing additional search engines is pretty trivial as well. If anything, Firefox provides more choice here than other browsers.