Good news - Firefox's free built-in VPN is starting to roll out for Firefox Android users today. If you're using Firefox on Android, you can now access practical privacy protection without needing to download another app. To turn the VPN on, tap the three dot menu > Settings > VPN (you'll need to log into your Mozilla account). If you don't see it available for you yet, please be patient as the feature rolls out.
More on Mozilla Support: https://support.mozilla.org/en-US/kb/mobile-android-vpn