たぶんこれかな
When entering Japanese, Firefox input both "Hiragana only" and "with Kanji"
https://bugzilla.mozilla.org/show_bug.cgi?id=1493428

韓国語IMEでも起きるらしい
firefox for android duplicates letters with some keyboards
https://bugzilla.mozilla.org/show_bug.cgi?id=1490391