This bug has been there for a while. In Firefox, when editing a post, hit 'enter' twice doesn't give you 2 blank line spaces. Despite it shows up on the editor correctly during editing, the updated post only shows 1 line spacing. And after you save the post and come back to the editor, you will find it shows only 1 line spacing now. The same problem occurs when you insert two lines and then switch to HTML editor and then back to rich format editing, the 2nd line dissapears. This problem occurs in latest version of Firefox and Chrome, but not in IE.
The other problem is much the same, when type two blank characters by hitting 'space' twice in editor, it only shows up 1 blank space in Firefox