KDE file picker not working on Firefox 88.0.1

I have used the KDE file picker on Firefox according to this post smoothly until a few months ago. Now, if I start Firefox from the menu GTK_USE_PORTAL=1 /usr/lib/firefox/firefox %u the file picker will not appear. Drag and drop works, but no file can be picked, either during uploading or saving.
If I open the terminal and just type firefox, it will work, but the GTK file picker appears.
It worked perfectly for a while when I first tried it, so I’m not sure what’s wrong now. The KDE file picker works perfectly on other programs.

Am I stuck with the GTK file picker?