HTML Editor as a Plug-In?
Found via Be Blogging, the Mozilla Composite Editor. Here's the scoop:
ComposIte is a chrome overlay which enables a streamlined Mozilla Editor for html composition in textareas. To use the editor, hit ctrl-e in a textarea. Alternately, you can turn on an 'Edit with Composite' button in the Composite prefs (v0.0.5 and higher).
The bad part, as Ugo notes, is that it does not generate XHTML. I haven't tried it, but it does come from Mozilla.org, so there's definite hope.