The actual problem seems to be caused by
https://bugzilla.mozilla.org/attachment.cgi?id=264215&action=diffPost
bug 116346 firefox
builds are sending
MIME Multipart Media Encapsulation, Type: multipart/form-data, Boundary: "---------------------------265001916915724"
[Type: multipart/form-data]
First boundary: -----------------------------265001916915724\r\n
Encapsulated multipart part: (text/plain)
Content-Type: text/plain; charset=UTF-8\r\n
Content-Disposition: form-data; name="update_check"\r\n
\r\n
Line-based text data: text/plain
Last boundary: \r\n-----------------------------265001916915724--\r\n
Pre
bug 116346 firefox
builds were sending
MIME Multipart Media Encapsulation, Type: multipart/form-data, Boundary: "---------------------------114782935826962"
[Type: multipart/form-data]
First boundary: -----------------------------114782935826962\r\n
Encapsulated multipart part:
Content-Disposition: form-data; name="update_check"\r\n
\r\n
Last boundary: \r\n-----------------------------114782935826962--\r\n
Looks like haserl is expecting Content-Disposition to be the first (or only?) part of the multipart form-data.