That has stopped working for me now too. It works just fine dragging and dropping from apps like XnView, but not to/from the desktop or Dolphin anymore.
I can’t recall the last time I used that feature. In any case if you can’t drag n drop to KDE’s open file dialog, it sounds like a KDE bug
I totally missed that. Thanks. I am downloading the iso now.
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YES
for me it started with an update to 129 or 130 (deb repo): DnD into FF broken.
as some other mentioned, sometimes an URL is pasted or nothing happens at all.
switched to flatpak (same) version … all fine
update to flatpak 131 … still all fine
switched back to deb version: same as above: DnD into FF broken.
Downside with flatpak version:
links from other applications (konsole ar any other) end up in nirwana.
next iteration:
DnD from FF into Dolphin or Desktop: fine
DnD from Desktop to FF: fine
DnD from Dolphin to FF: does not work at all or just a path is inserted
Deb variant (from Mozilla)
Probably have to set the Firefox flatpak as your default browser in plasma’s systemsettings.
you have a point here! 
personally i find it quite interesting and challenging to find the root causes of this issue.
latest twist: DnD into FF does work from when dragging files from desktop, but not when dragging from Dolphin.
It does not work from the desktop for me, nor dolphih, but does work from regular apps like XnView which I assume is based on GTK (I should mention this is in X11).
XnView and GNOME Nautilus as well
Kfind (Qt based) works fine for me also