I've seen this in the past but don't remember the cause and solution. Samsung had this issue once, and from what I've seen on the internet, the solution was to disable Android web preview, but that's not working. This problem just started on my Moto G4 Android, only in Firefox, only on certain websites where text is entered into forms. It isn't happening on this website. It isn't happening in Chrome or other apps. I've cleared temporary data, incuding cached storage, in Firefox and all but data in the phone Apps for Firefox, disabled options in settings, restarted phone, but none of that worked. Example: if i begin to enter, let's use "Walking is good," the result is "Walking is goodW." If I am swiping to enter, the result is "is goodWalking." The error causes the affected text to reappear in front of the cursor. Highly inefficient and a real time killer! I also noticed that if I enter a space in the first position the error does not occur unless I backspace over the space and continue typing.