How to permanently disable update notifications in Firefox

I have automatic updates turned off in Firefox, however, the browser will periodically pop up a window in the top right corner, upon firefox startup, asking me if I wish to update. How can I permanently stop this window from appearing? This pop-up window is a problem because the situation is occurring on the PC of my mother, who is elderly. If she accidentally closes the pop-up in the wrong way, by clicking on the "update" option, the next time Firefox starts it will attempt to update and open a new Windows security window asking for an administrator password, which she doesn't have. This leads to confusion and she is then unable to operate the PC, which can be a health and safety risk. tl;dr : Please add an option for permanently disabling this update notification pop-up window because it is a safety risk.