One of my favorite things about running KDE’s own distro, KDE Neon, is how it defaults to using the KDE file UI in Firefox. I’m not sure why Kubuntu and Fedora KDE don’t know this, but it’s easy enough to configure.
The magic behind this is Firefox’s ability to use an XDG desktop portal. You probably already have the KDE XDG desktop portal installed, but if not, simply run:
sudo dnf -y install xdg-desktop-portal-kde
sudo apt install xdg-desktop-portal-kde
Configuring Your Shell
Next, we tell GTK apps, like Firefox, to use the KDE file dialogs if they’re able. We do that by setting in a variable in either our ZSH or BASH configuration (whichever your default shell is).
echo export GTK_USE_PORTAL=1 >> .zprofile
echo export GTK_USE_PORTAL=1 >> .profile
Now just log out and back in and you get should native KDE file dialogs when saving or opening files with Firefox!