Warning about what? Today Mull is still likely the most (non-TOR) privacy respecting Android browser available. It was never the most secure as that would go to a Chromium fork. Until the next Firefox-Android updates come out, Mull is still the browser to use for privacy, and we all have time to evaluate alternatives. Just because you haven’t yet found someone to provide you a privacy respecting browser for free doesn’t mean that the tools to attain this aren’t available to you.
Check the recommendation here Divest OS Apps Discontinued - #12 by Cue