I have found answers in other discussions:
- Self-Distributing add-on instalation
- Why can I not load a simple custom extension/addon xpi file in Android Nightly?
At the moment Firefox for Android only supports installing add-ons directly from addons.mozilla.org; attempting to install an add-on from another site will end up downloading the XPI file. That said, there is a developer/poweruser feature that allows users to install a locally saved XPI file. See this Reddit thread for details.