This is a usability issue.  When using Opera and visiting a page like
<URL:http://packages.qa.debian.org/e/etckeeper.html> with links to a
patch file (the ubuntu patch in this case), Iceweasel fail to show the
patch when I click on the link.  Instead it show a popup asking me to
either save the file or pick some application to display it with.
When I do the same with Opera, the browser display the text file, and
I would like Iceweasel to do the same.

The content type of the patch is text/x-diff, as can be seen from this
demonstration:

  % HEAD http://patches.ubuntu.com/e/etckeeper/etckeeper_0.48ubuntu1.patch
  200 OK
  Connection: close
  Date: Fri, 15 Oct 2010 09:36:19 GMT
  Accept-Ranges: bytes
  ETag: "1e5e0e2-2068-48c4d15906e40"
  Server: Apache/2.2.8 (Ubuntu) mod_python/3.3.1 Python/2.5.2 mod_ssl/2.2.8 OpenSSL/0.9.8g
  Content-Length: 8296
  Content-Type: text/x-diff
  Last-Modified: Mon, 26 Jul 2010 16:43:29 GMT
  Client-Date: Fri, 15 Oct 2010 09:36:19 GMT
  Client-Peer: 91.189.90.149:80
  Client-Response-Num: 1

Please change Iceweasel to show text/x-diff files inline as a text
file.

I experience this problem with Debian/Lenny, but tested it in Squeeze
with version 3.5.13-1 and the problem still exist there.