Luckily, I finally found the solution. The problem is related to the so-called plugin container. Firefox
had activated a plugin named evince. I simply deactivated it, and everything works as usual.
The annoying point about this: I never installed any plugin for firefox / conkeror other than flash and
vlc. The reason of the above described problems was apparently that, some time ago, I installed evince.
In the sequel, firefox activated the evince plugin. This might have happened when I once tried to open a
ps file under firefox (not conkeror).
I guess this is not a bug of conkeror, but of firefox as well as of palemoon. In firefox, the "evince
plugin" does not work for me either, trying to open a ps file leads to a crash. For palemoon, the same
issue as discussed here is described at https://bbs.archlinux.org/viewtopic.php?id=214285.