Seems to be a problem with the encoding/decoding of multi-part form data. The POST source shows:
Content-Type: multipart/form-data; boundary=---------------------------265001916915724 Content-Length: 464 -----------------------------265001916915724 Content-Disposition: form-data; name="comment_text" <p> test by Denes using Firefox 8.0.1 on Win XP</p> -----------------------------265001916915724 Content-Disposition: form-data; name="_formkey" 51d0905f-1ba0-4f3d-96c2-5dc38178dd36 -----------------------------265001916915724 Content-Disposition: form-data; name="_formname" article_comments/create -----------------------------265001916915724-- but Firebug only lists two parts: Parts multipart/form-data _formkey 51d0905f-1ba0-4f3d-96c2-5dc38178dd36 _formname article_comments/create