Radio turning into checkbox in Firefox browser

I have following simple html code. 3 radio buttons followed by a textbox with placeholder attribute and then followed by a checkbox. When viewed in firefox browser, the 3rd radio keeps turning into checkbox.

1) On mouse over on radio, it reverts back. 2) On removing placeholder attribute on textbox, it goes away. 3) On removing checkbox, it goes away.

Has anyone faced this issue ? is it a firefox bug ?

    <input name="source" value="1" checked="checked" type="radio">Radio Button 1<br>
    <input name="source" value="2" type="radio">Radio Button 2<br>
    <input name="source" value="3" type="radio">Radio Button 3

    <br><br>
    <input name="mybox" size="50" maxlength="50" type="text" placeholder="placeholder">

    <br><br>
    <input name="mycheck" value="true" type="checkbox">Checkbox

JSFiddle Link - View In Firefox