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.
Scan to folder usually doesnt involve a PC at all. The scanner saves the file on a network share via NFS or smb