I was curious if trying to load it via file:///storage/emulated/0/Download/... would work (as my recollection is that .xpi installation is content-type: sensitive) but insult-to-injury is that FF Nightly for Android searches for the string "file:///storage...", so they seemingly have nuked even the file: protocol handler for Android. Good times over there at Mozilla

That's not true anymore. You have to press the Firefox logo on the about screen a few times, which will make the menu option appear in settings to install an extension from the local filesystem