Why does file:// not work?

I've got an HTML file on my phone, and would like to view it. But Firefox on Android has neither a menu entry "Open File", nor does using the file:// method work, nor is any app associated with the .html file extension.

BTW, Chrome suffers from the same problem. And it is not an issue with the file, it works when opened via the https:// method from my home server.

Why, oh why, can't I open a local file on Android?