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).
To be an Android Firefox user, you already went to the trouble of installing an alternative browser on Android. The motivations to switch browsers might as well be the same as those which will push you to switch search engines.
I would totally expect a reasonable number of Android Firefox users, of all apps, to change at least the search engine.
Chrome is embedded in Android way more than people realize. It's at a level of what IE was with windows, probably way worse too depending on how you look at it.
DDG search (same setup, in Android Firefox) is quite slower for me than Google search, it needs a few good seconds to bring the search results. No idea why this might be.
The concern is that google basically funds firefox, and can choose to revoke that funding at the most inconvenient time for mozilla, risking bankruptcy. Companies ebb and flow on cash flow, and a unexpected drop at the exact wrong moment can cripple even the most well funded ones.
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).