When using the virtual keyboard, IME (no matter third party's or Windows 10's) dosen't work at all. In this case, ONLY English input works. other languages (such as Chinese IME and Japanese IME) inputs are not working. This issue only happens on the virtual keyboard, not on physical keyboards. I'm recently using a Windows 10 tablet equipped with a touch screen, and sometimes I remove my detachable keyboard and use the touch screen only, and this issue stops me from using Firefox without my detachable keyboard. Firefox version is Firefox Quantum 69.0.1 (64 bit).