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
Pretty sure file:// is very broken in different ways on every android browser.
For example, on kiwi browser typing in a file URL causes it to be searched, but using the "go to URL in clipboard" button (with the file url in your clipboard) works. Except when you randomly run into some weird android file permission issue and the browser just can't see certain files...
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