I use FCKeditor in websites that I develop for some friends (I'm NOT a full-time, web-development professional) so that the owners of those sites can edit their own content with the formatting they wish to use.
Recently, I redesigned one of those sites to take advantage of CSS instead of using tables, and that has changed the way in which the formatted content created within FCKeditor appears. Simply stated, when they format something using the FCKeditor page I set up for them (which still uses tables), the resulting content gets mangled so that some parts of the text overlap each other, or are otherwise out of whack. The best way I can demonstrate this is by some screenshots.
Here is how the text looks within the editor (it's the same for Firefox or IE):
Here is how it looks in the CSS/DIV-based, final page in Firefox:
Here is the same page in IE:
You can see just how much it gets messed up. In case it may help, here is the CSS declaration for the DIV being used to contain that text:
div.lcomments {position:absolute;top:800px;left:0px;width:700px;text-align:center;background-color:#FFFFFF;z-index:3;}