User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:109.0) Gecko/20100101 Firefox/118.0

Steps to reproduce:

1. Do a clean install of Firefox - do nothing to modify Firefox except use about:config to set the XDG settings to 1 to enable the KDE file picker instead of the GTK file picker.
2. Run Firefox, browse to an image file on the Internet.
3. Do a Save Page As which opens the KDE File Picker.
4. Note: This is running on openSUSE Tumbleweed Linux Snapshot 20231012 using KDE Plasma Version 5.27.8, KDE Frameworks Version 5.110.0, QT Version 5.15.11, Kernel Version 6.5.6-1 default 64-bit, Graphics Platform X11 running on a Rzen 5 2600X CPU with 64GB RAM.
5. Note: Bug report was submitted to KDE: They responded as follows:

Nate Graham 2023-10-16 18:10:00 UTC
Can reproduce. However it doesn't happen in our reference portal test implementation, which means it's being disabled in Firefox's code somehow. I'd recommend that you submit a bug report to them. Thanks!

Actual results:

The Show Preview Pane option is greyed out and neither a click nor F11 can activate it.

See attached image.

Expected results:

The Show Preview Pane should be available for activation by a mouse click or F11.

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Gtk' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Looks like portal KDE file dialog. Which pref do you modify at about:config to use KDE file dialog?
Thanks.

I used the following XDG settings in Firefox:
widget.use-xdg-desktop-portal.file-picker set to 1
widget.use-xdg-desktop-portal.mime-handler set to 1
widget.use-xdg-desktop-portal.settings set to 1

I based these settings on numerous Internet posts explaining how to change the picket from GTK to KDE.

These settings were left as they were as they seemed to have nothing to do with the file picker per se:
widget.use-xdg-desktop-portal.location is set to 2
widget.use-xdg-desktop-portal.open-uri is set to 2

This is the corresponding bugreport on KDE's side: https://bugs.kde.org/show_bug.cgi?id=475668