Even if you do, does it actually change that much? You cant actually stop the os on mobile from using its default browser for most of it's tasks. Firefox on iOS is basically a reskin of safari if I am reading it correctly.
On Android, the only thing that the default browser ever opens for is captive portal detection. Otherwise, all links from apps like Signal or OSMAnd open in Firefox. Granted, I am running Lineage OS. Does a phone with Google Play Services installed override Firefox for Android to use the preinstalled web browser instead?