Quite puzzled at how this can happen, but try selecting some text in Firefox, it starts randomly deleting whitespace. Also, I noticed that sometimes when you enter the editor, no text is shown until you select something or switch the color scheme.
When you enter the editor and no text is shown, which I know exactly what you mean, it is usually caused by CodeMirror being loaded in a height:0 element. A quick fix might be to use editor.update() on a time out after animation.
Now noticing strange behavior in FF; very strange; investigating.