So, you know the full path to the mountpoint, so you can type it in the name field.
So, can’t you just go to /mnt/DAB6E106B6E0E44B in the dialog box?
I tried typing in the full path and it worked.
So I guess no issue on the mounting, but more of the dialogue window?
Exactly. Mark my post above as the solution, please 
Get used to crappy dialogues or try with the xdg-desktop-portal-kde…
I will try this.
Why do I get the GNOME dialogue tho? Isn’t supposed to be Dolphin?
Because Firefox is like that… Everyone gets GNOME (GTK) dialogues on Firefox, unfortunately. There is a hack to replace these dialogues with native Qt dialogues used on Plasma, but it’s a hack – it is usable, but a bit buggy.
I just installed xdg-desktop-portal-kde
How do I set the environtment variable GTK_USE_PORTAL=1 ?
Install the following packages:
sudo pacman -S xdg-desktop-portal xdg-desktop-portal-kde
Now test that it works, by running Firefox from the command line like this:
GTK_USE_PORTAL=1 firefox
If it does, you can set it permanently in two ways:
Either edit the .desktop file for Firefox and just add GTK_USE_PORTAL=1 to the exec line.
Or alternatively, create a script file in /etc/profile.d/ with
GTK_USE_PORTAL=1
This will set it for all users for all applications (may or may not be what you want).
You can do it in one line, like this:
echo "export GTK_USE_PORTAL=1" | sudo tee /etc/profile.d/gtkportal.sh
And then reboot. If you get sick of it, just delete the file above:
sudo rm /etc/profile.d/gtkportal.sh
I followed all of your instructions and it now uses the qt dialogue.
I’m not noticing any weird things so far.
Thanks!