Not related to Firefox, but more of a new product idea.
I'm not aware of any keyboard app that could be considered as private by design. It seems the market for this is dominated by Microsoft with SwiftKey and Google With GBoard. It would be good if Mozilla made a keyboard app that was private but still had the features of the most popular keyboard apps.
There Is Openboard in Android, you can install it on the play store or with F-Droid, fully open source and has all the basics features
I use Hacker’s Keyboard. I hope the new kb will support 75 % layout or at least 60 % (it doesn’t need to be default layout) with special keys and NumPad hidden under Fn key (arrows should be visible at all times). Even Hacker’s keyboard is missing some keys (AltGr, Right Ctrl, Meta/Super/Win, Caps Lock) and Left Ctrl is in place of Caps Lock and Esc in place of Left Ctrl.
Also, do support multiple locales (sw layouts), like English (UK), Slovak (QWERTY), Hebrew, Greek. We should be able to switch between these layout by swiping Space.
And it would be awesome if we could switch between alphanumeric and numeric layout at our will using an option hidden in a button.
And buttons like Ctrl, Alt, Shift should use indicators on their buttons if they are pressed or not.
It might a good idea to customise the layout, like what chars should be shown when long-pressing a button and it what order (e.g. for `a`, this might include `á`, `à`, `â`, but also `א` or `α` if a user wants them to be shown).