Comment 52 for bug 133133

Not entirely sure if this is related, but I had the exact problem described at http://bbs.archlinux.org/viewtopic.php?id=69035 and the solution there fixed it.

What happened there was that firefox was using EasyTag instead of konqueror/dolphin.

Now, while this is slightly different, the fact is that firefox seems to be using /usr/share/applications/mimeinfo.cache for information about file associations, because when I removed EasyTag from there, Firefox started using Dolphin.

Oddly, the next option (which I didn't delete) was Thunar, but this, apparently, was ignored.