Does sudo snap connect firefox:removable-media not fix the problem? There’s a GUI for this in GNOME Software in Ubuntu 18.04 (which will be backported) and you can request an autoconnection for it (and tag @osomon) in #store if you’d like to have this done automatically for users in the future!
In fact, LibreOffice is already auto-connected for this. If you’ve already installed LO you might need to manually connect it though? I don’t quite know how that works. Also please can you file topics/bugs in future to get these things fixed? 