I'm running the latest version of WordPress 3.0.1 and Firefox 3.6.8 on Mac OSX.
When I'm editing a post the normally resizable post field and excerpt field are not resizable in Firefox 3.6.8 under certain conditions.
Main post field: When in HTML mode the box is not resizeable, when in Visual Mode it can be resized.
Excerpt field: In both HTML and Visual modes the box cannot be resized.
When using Firefox 4.0 b2 the fields are resizeable but FF beta is not quite stable enough to do tons of work with.
The fields resize properly with both Chrome and Safari but Safari has too many bugs and Chrome is a pain to use in workflow.