Hi, the code is all open source and freely available on GitHub the link is available on the add-on page. Key presses are not recorded, the mouse press detects what letter you have pressed and immediately returns that letter to add it to the input field that has focus, there is no input inbetween that captures your data.

I built this for the company that I work for as we use it in our own Kiosk mode software that utilizes Firefox.