I'm not well versed in Android changes, this is just the conclusion I came to when Nougat came out and haven't tried it again since. If there's an easier way to use MITM with Android without modifying the APK / rooting your device, I'd love to know!
[1]: https://github.com/NVISOsecurity/MagiskTrustUserCerts
[2]: https://github.com/sensepost/objection/wiki/Patching-Android...
With root access, you can put your certificate in the root store, which most apps use for validation. I wrote a blog about it around the time Android 7 came out. Some apps (either not enough or too many, depending on your perspective) leverage Android's certificate pinning feature to connect a domain to a specific TLS certificate. You can't MitM those apps without either modifying the APK or hijacking them via Frida.
Lastly, if you want to MitM Firefox, you need to enable their secret debug menu (available on some builds) to enable the flag that enables user certificates. Flagging the relevant setting in about:config does not work and about:config isn't even accessible on stable builds of Firefox for Android.
Sadly, there's no good way to MitM an Android device without full system access. Android has always been quite annoying to use custom CA certs for, but since Android 7 it's pretty much impossible without root access. I'm not entirely sure why they went this route instead of sticking to their existing warnings (i.e. a notification every time a custom CA gets used, like in Android 4, and a constant "your network may be monitored" label in the notification tray like in Android 5+).
Thanks for the Firefox tip as well, it's been a while since I used Mitmproxy and I see things have not become easier since.