This would be great for a locally-developed website, especially for developers who can't afford or don't want to host their site elsewhere!
I have a collection of HTML files on my tablet's storage that I access regularly without turning on wifi since it saves battery, I don't always have wifi, and my tablet is slow. It is a hack, and only Firefox does it, which I am thankful for. And it only works on old Android versions (9 maybe.? Lol). You have to hook up your device to a PC running Firefox and then remote debug your device and set the URL from your PC to the correct file. And never close that tab after that!
I guess another workaround is to run a local HTTP server.
Every other Android browser supports some form of the file:// or content:// protocols. Like Firefox used to before the rewrite. Come on Mozilla. As long as you disable these from being set on window.location, what's the security issue?
Firefox stands for free software. Isn't one of the reasons of free software to motivate people to write their own software? I wrote lots of software in html files with javascript. With my old smartphone and a some month old Firefox it is possible to open local stored html files, because before you reduced firefox some years ago I stored the links to local html files in the menu item "Sammlung" (german language) - maybe it is collection in english language. With my new smartphone and newest firefox, I can't open local stored html files. I can't even use a local stored start page in firefox.