Firefox is not the default browser on mobile OSs. 99% of users don’t change the default. Most people don’t know that this is possible, desirable, or an issue.
Posting from Firefox on a desktop. I like my Ad Blockers to actually work.
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?