I think it's android that won't let Firefox access the local files. Doesn't work with Fennec either

Android file permissions. Android grants access to a specific subdirectory for each app. So u need to go to file:///somedirpath/urfile.html

U might also be able to grant full file access to ff but I would say that's a huge security risk. Alternatively u could serve the file dir with python from termux.

I was pretty irritated when I noticed that myself. I'm guessing that there's some form of attack or class of attacks that this is designed to avoid (e.g. IIRC a webpage opened from a file:// URL can have Javascript access files via file:// as well, and I imagine theoretically could upload them somewhere or something), but it sure is goddamn obnoxious.

My guess is that you can probably fire up a local HTTP server on your phone and view it via that. I don't think that Android phones set up a firewall by default, so have it listen on a loopback address, like 127.0.0.1.

Based on this, you can use a file manager to share the HTML file with Firefox.

The Android way to do this is to open the file from some sort of file browser app and select Firefox from the list of apps that registered for that file type. Unfortunately it looks like Firefox doesn't register itself for html files.

It's clunky, but I can open files in firefox by using a file browser app (I use x-plore), selecting 'open with', then selecting firefox. Sometimes it's not in the list, but there's a selector for what type of file (text, video, audio, '*'). '*' lists all the apps.

Sometimes stuff still refuses to open, but things like pdfs and html files usually work

I personaly use this extension if i need to open html