Fwiw it's the app's fault when that doesn't work, not Firefox's or Android's. (I too encounter it frequently and it's annoying. I still have and use the Lockwise precursor app, because it's easier to copy from than Firefox when this happens.)
have you tried firefox sync? recently gave it a try and on android it works seamlessly. Cross browser integration is an extra step though.
Yes, that's what I meant. I use two PCs and two Android phones and Firefox Sync keeps my passwords across all devices.