Custom Browser support for AppAuth for iOS has been implemented. Here's how to configure AppAuth to use a custom browser:
First, add the following array to your Info.plist (in XCode, right click -> Open As -> Source Code)
<key>LSApplicationQueriesSchemes</key>
<array>
<string>googlechromes</string>
<string>opera-https</string>
<string>firefox</string>