Hello.

I was trying to test this feature, to see how different was from the normal (gtk) file dialog firefox uses.
After some steps the program works, but besides the instructions provided in the readme.md file, this was needed:

create portals.conf inside ~/.config/xdg-desktop-portal (xdg-desktop-portal folder was created too).

Inside portals.conf:

[preferred]
default=gtk
org.freedesktop.impl.portal.FileChooser=lxqt

After following the steps found in the README.md and doing this, rebooting and launching firefox makes the function to work properly.

Summarized steps:
Install xdg-desktop-portal-lxqt

In about:config inside firefox, set:

widget.use-xdg-desktop-portal.file-picker = 1
widget.use-xdg-desktop-portal.mime-handler = 1

Create the file and folder detailed previously.
Reboot the system and firefox should use the lxqt file manager (pcmanfm-qt)

Info:

OS: Arch Linux
Firefox version: 119.0
LXQt Version: 1.3.0

Maybe the file is generated in other distributions, i have not tested that. Just wanted to mention this for those in the same situation.

Bluey.