Hi,
I noticed that the performance of the font size drop down in Firefox degrades if the content of the editor is sufficiently large.
You can try it with these steps:
1) Go to the page with the demo editor.
2) Ctrl + A to select the content, Ctrl + V a few times (5 - 8 times should be sufficient to demonstrate this).
3) Ctrl + A to select the content.
4) Click on the Size dropdown. It should take a while before you get a response.
Works fine in IE.