Firefox Quantum : HTML files from internal storage (/usr) do not open

It seems that you have installed Firefox as a Snap. It is normal that it can't access the folders inside root-filesystem:

$ snap interfaces | grep firefox
:avahi-observe             firefox
:browser-support           firefox:browser-sandbox
:camera                    firefox
:cups-control              firefox
:desktop                   firefox
:desktop-legacy            firefox
:gsettings                 firefox
:home                      firefox
:network                   firefox,pulsemixer
:opengl                    firefox
:pulseaudio                firefox,pulsemixer
:screen-inhibit-control    firefox
:unity7                    firefox
:upower-observe            firefox
:x11                       firefox,pulsemixer
-                          firefox:network-observe
-                          firefox:removable-media

Consider to remove Snap with

sudo snap remove firefox

and install APT version

sudo apt install firefox

and visit local help page.

You seem to have an extra forward-slash in your file path.

Use file:///usr/share/texmaker/usermanual_en.html, it should work fine.

Edit- Just in case, here's the online help repo of TexMaker http://mcs.une.edu.au/doc/texmaker/usermanual_en.html