Google doesn't believe in privacy and they don't want you to be able to block tracking. Just look at the Android permissions system for an example.
Also, Firefox is generally faster than Chrome from a user's perspective, because Chrome is optimized to send you to the Google Search results pages where you're supposed to click cleverly-disguised ads on your way to your destination. In contrast, Firefox is designed to send you directly to where you want to go. Try it for a while to see what I mean.
(I hope that someone will write a script to compare the amount of time wasted by Chrome's pushing people to Google search results vs. the fast search experience on Firefox where ctrl-l gets you fast, accurate suggestions and ctrl-k gets you search suggestions.)
Also, Firefox for Android is the only mobile browser that lets you use add-ons, and adblockers can save over 70% of bandwidth. Self-destructing cookies works on the mobile version too.
Why are people who are concerned about privacy using a browser created by a company that doesn't believe in privacy? You have no privacy options with Chrome on Android (at all) either. Firefox offers a lot of privacy protection out of the box, and you can use add-ons on the mobile version.