Firefox for android should block access to saved passwords after a new fingerprint is enrolled on Firefox Android app and asks for the Firefox Account password instead.
This is specially important when someone else gets my phone's PIN (by leak or theft) and enrolls his own fingerprint to gain access to my Firefox saved passwords.
I've tested with other apps and there are many of them that actually invalidate my fingerprints after a new one is enrolled. Besides, I googled it and found what seems to be the an Exception to look for on that situation: java.security.InvalidKeyException.
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Firefox does not store or check the fingerprint. This is handled by the OS and the validity or invalid result is forwarded to Firefox. We can't do what you are asking.