I may be wrong but using wireshark you can see that you receive the next line:
'Content-Type: text/html'
which says to the browser it's a html page.
So it's a problem of that particular forum, but to discard it... can you
provide us another link in which is happening the same?
** Changed in: firefox-3.0 (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/275156
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