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