Hi With most Android devices shipping with the Google Play store, it is the easiest way to securely install Firefox for Android. If you wish to install the .apk, head to https://archive.mozilla.org/pub/fenix/releases/ and install the most recent version for your device. To do this you will need to turn off an important control that projects your device from malicious apps. You may also find that your app does not correctly update. I hope that this helps.