Firefox Quantum installed but doesn't see internal or external storage drives [duplicate]

As you installed Firefox as Snap it is expected behaviour.

By default it has access only to the home folder. See this question and answer for details. And it will produce permission error on access to /media/$USER/drive/ folder:

You should manually connect :removable-media plug to Firefox to remove this error

snap connect firefox:removable-media

and you will get the following connectors list:

$ snap interfaces  | grep -v ".*-$"
Slot                       Plug
:avahi-observe             firefox
:browser-support           firefox:browser-sandbox
:camera                    firefox
:core-support              core:core-support-plug
:cups-control              firefox
:desktop                   firefox
:desktop-legacy            firefox
:gsettings                 firefox
:home                      firefox
:network                   firefox
:opengl                    firefox
:pulseaudio                firefox
:removable-media           firefox # <- it should see drives in /media/
:screen-inhibit-control    firefox
:unity7                    firefox
:upower-observe            firefox
:x11                       firefox
-                          firefox:network-observe

If it does not work or you do not want to make such connection - remove Snap version with

snap remove firefox

and install deb-version instead (you will get 60.0.2):

sudo apt-get update
sudo apt-get install firefox