I'm not sure which feature you mean. On desktop, Firefox marks misspelled words with a squiggly red underline. On Android, this is disabled by default. On Android, when you tap on a form field, Firefox uses the keyboard settings from the system, so your general auto-correct setting kicks in. To modify that, you could try here: (Android) Settings > Language & input > [keyboard] > Text correction That may vary depending on your version of Android.