Before Fenix (i.e: Firefox Android 68.x), Firefox Android was able to retrieve phone's storage using file:// URI. Now, in the current version of Firefox Android, if you attempt to access a file:// URI (example: file:///storage/emulated/0/ ) it will display a blank screen.
Chrome and Chromium-based browsers currently still support file:// URI.
A workaround for that (current version of Firefox Android) is to install a HTTP server app on Android (example app), open Firefox Android, and enter the displayed URL on the app like http://localhost:[port]/ , http://127.0.0.1:[port]/ or http://[Local IP Address]:[port]/
Adding support for file:// URI is helpful/useful for web developers who need to open an HTML file in Firefox Android (either with experience or learning from it), especially for those with broken desktop and/or laptop computers. Affected extensions include SingleFile (compatible in Firefox Android, but not available in the list of Add-ons), which saves web pages as HTML and open them via file:// URI.