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 — 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.
Thank you. I'll use that information about the USB ID for my report to KDE. However, could you elaborate somewhat? That is, would it be correct to ask for Plasma to not merely utilize the USB ID to identify whether a device is accessible, but test whether it can be accessed by MTP first, before presenting that option to the user? Regardless, per what you've said, I've consolidated my previously disparate reports under https://bugs.kde.org/show_bug.cgi?id=486931.
Additionally, I'll definitely mention on Bugzilla that incorporating some of the patches in that package would be a feasible implementation method. Relevantly, I'd like to use that package, but because I'm not using Edge, would adding that repository do any harm (for instance, would it be added with higher priority by default than existent repositories)?
I don't think reporting the USB ID thing to Plasma is useful and will go far - for 99% of users (that use some kind of Android/AOSP) the modus operandi is fine and helpful. With many Android devices and OSes requiring you to do something on the device after plugging it in, testing does not seem to be feasible to me.
There's no need to add the edge repo, as the latest release of mobile-config-firefox should be in v23.12 by now (it's been updated there since my last post).
The command I posted does not add the repo, but only uses it for the one package without adding the repo permanently.