Firefox uses Spellchecking (
https://support.mozilla.org/en-US/kb...-spell-checker), Auto Form Fill (
https://support.mozilla.org/en-US/kb...ly-fills-forms) and Adress Bar Autocomplete (
https://support.mozilla.org/en-US/kb...mplete-firefox). It could be either one of those three, depending on where in the browser or on the page you are typing. Have a look at these three information pages as they can explain how to customise the feature to do what you want, or switch it off.