Firefox was still using widget.use-xdg-desktop-portal.file-picker=2, but I'm using my default profile. I had not changed it before, so I'm not sure if it was supposed to change the value on update or not. Changing it manually to 1 made it show the KDE file picker correctly, though, and it seems to work correctly too. Tested on Plasma 5.26.0 (X11).
I had some slight problems with it generating sync conflict files between PC and phone, but after forcing syncs from both sides it stopped. It still synced correctly and I'm not sure how reproducible it'd be (and I found nothing similar in the bugtracker upstream, so it might just have been something local), so I'm leaving a +1.