You can't install a root CA in Firefox for android.

You have to install the cert in android and set Firefox to use the android truststore.

You have to go in Firefox settings>about Firefox and tap the Firefox logo for a few times. You then have a hidden menu where you can set Firefox to not use its internal trust store.

You then have to live with a permanent warning in androids quick setting that your traffic might be captured because of the root ca you installed.

It does work, but it sucks.