The curse of the default tells us that this will never be a problem. Do you honestly believe that 99% of the Android Firefox users spend time to change the settings on the browser? (change to DDG, block all 3rd party cookies, etc.)

I did a quick search on DDG but no meaningful results came up with search engine for firefox on android. Gut feeling tells me 'minimal'. People do like their google remembering stuff. Convenience is the enemy of security (privacy in this case).