Assigning a font whose name contains spaces (e.g. via the Font panel) leads to HTML like this:
<font class="Apple-style-span" face="'Arial Black'">Text</font>
The additional quotes around the font name lead to problems with some browsers, e.g. Firefox.
Fixed in http://trac.webkit.org/changeset/109382.
*** This bug has been marked as a duplicate of bug 79448 ***