This worked for me on Firefox 97.3.0:

1. Install the self-signed CA certificate into Android (for me, this was under Settings -> Biometrics and security -> Other security settings -> User certificates). My certificate had to be DER format.
2. Open up Firefox Settings -> About Firefox
3. Click on the logo 5 times (until "Debug menu enabled" popup appears)
4. Go back to Settings - there should be a "Secret Settings" option now. Choose that.
5. Enable "Use third party CA certificates"

This worked for me on Firefox 97.3.0:

1. Install the self-signed CA certificate into Android (for me, this was under Settings -> Biometrics and security -> Other security settings -> Install from device storage -> CA certificate). My certificate had to be DER format.
2. Open up Firefox Settings -> About Firefox
3. Click on the logo 5 times (until "Debug menu enabled" popup appears)
4. Go back to Settings - there should be a "Secret Settings" option now. Choose that.
5. Enable "Use third party CA certificates"