The case remains completely mysterious ...

My configuration is exactly the same (archlinux, firefox 47.0.1-1, and conkeror-git
1.0.3.r11.g66bcc3f-1), I do not use any .conkerorrc, and I start without the .conkeror.mozdev.org
directory. To repeat the story with a specific example: I visit http://arxiv.org/abs/1604.06973, I click
on the link "PDF", which points to the pdf-file http://arxiv.org/pdf/1604.06973v1, and conkeror crashes.
I may instead press "s" ("save"), choose that same pdf-file, and download it -- and its name will be
"1604.html". The erroneous extension "html" is of course added by the browser.

Always downloading a file instead of following the link to it is, unfortunately, no solution. For
instance, when visiting pages with an embedded pdf, conkeror used to show a black area where the pdf is
supposed to be shown, now it crashes immediately when I visit such a page.

Thanks anyhow for the feedback. I guess I will not find out what the problem is, it has occurred since a
couple of days, after a usual upgrade of the system, which included firefox but not conkeror. But as
noticed already, the upgrade of firefox is not likely to be the problem, I have the same behaviour with
firefox 45 and with palemoon.