I recommend trying to use KDE Connect (or scp, rsync ... another network based way) to send the screenshot from the phone to your other computer instead.
MTP/other file transfer protocols do not (yet?) work with mobile Linux, so this failure is to be expected. It only shows up for connection, because if your device ran Android, it would be an option โ AFAIK, Plasma acts this way because of the USB ID of the device.
Also, regarding your main issue: While you can report this to Mozilla, please be aware that Firefox is being "patched" to better work on mobile by https://gitlab.com/postmarketOS/mobile-config-firefox. I suggest you to install the latest, not yet in postmarketOS 23.12 mobile-config-firefox package from edge first by running:
sudo apk del mobile-config-firefox
sudo apk add mobile-config-firefox --repository http://mirror.postmarketos.org/postmarketos/master
While it may not fix every issue possible, it should improve the experience.