Publishing a Mozilla-signed XPI outside of AMO is a breach of ToS, and can lead to being removed from the AMO.
Self-hosting xpi files is definitely allowed, and all xpi files are signed by Mozilla. https://extensionworkshop.com/documentation/publish/self-dis...
See https://extensionworkshop.com/documentation/publish/signing-...
My previous statement was overly broad, but the restrictions do apply to rejected xpis.