In the Firefox 41.0 version from the dext5 editor, When you enter the Hangul text after changing the line, the curser moves to the previous line. After certain amount time has passed, the cursor is moved forcibly to the first row. This phenomenon can occur in any HTML.
In Firefox 41.0 version, The cause of the error of Hangul input function is from the collision of the TSF(Text Service Framework is new IME API.) function of MS that has been provided since Windows Vista.
Please upgrade to the Firefox 42.0 version. (No automatic upgrade support)