Comment 28 for bug 1551171

Comments 16–20 are ancient; bug 713802, to enable GIO by default, was marked FIXED over a year ago.

tl;dr of comment 24: if you're on KDE, and Firefox isn't opening e.g. PDFs or directories with Dolphin or Okular, it's neither a Firefox nor a GIO problem. KDE supports a custom priority property in .desktop files, and built-in KDE apps have it set by default so they win out over everything else /if you haven't specified otherwise/. (Without this behavior, even KDE could start opening PDFs in Inkscape, because you never explicitly asked to use Okular. I'm not sure how GNOME et al. avoid a similar problem.) Meanwhile, GIO doesn't understand any of this because the property is non-standard.

If you manually edit your file associations, KDE *will* write out your choices in the standard way, and GIO will obey them.