Use Apps That Work Together
Apps can launch other applications, so you can take advantage of this to build ecosystems of apps that work well together and use each other as the defaults.
Google’s iOS apps are the star of the show here. Let’s say you have Chrome, Gmail, and Google Maps installed. When you tap a link in the Gmail app, it will open in the Chrome app. When you tap a map link in Chrome, it will open in the Google Maps app. And, when you tap a business’s email address in the Google Maps app, it will open in the Gmail app.
If you want to change default apps because you really prefer Google’s services, try just using as many of Google’s apps as possible. They’ll work together and allow you to avoid the standard iOS apps as much as possible. If you use third-party apps, hopefully they offer support for choosing your own preferred apps.
In case of Firefox I haven't heard or done anything of this sort.