Contrary to what somebody wrote, Fennec is not one major version behind and builds upgrades very frequently. Please, see https://f-droid.org/en/packages/org.mozilla.fennec_fdroid/
Please, don't reinvent the wheel or let Mozilla work too much outside of its scope. The best FOSS keyboard out there IMHO is https://github.com/AnySoftKeyboard/AnySoftKeyboard and better redirect some resources to that project.
