There is an X resource Xft.hinting, with corresponding XSETTING Xft/Hinting, and corresponding xfconf property /Xft/Hinting in the xsettings channel, which controls whether hinting is enabled. It is a boolean on/off with -1 meaning "default", similar to Xft.antialias. xfce4-appearance-settings has a combo box to control hinting style, which sets the /Xft/Hintstyle property, but nothing ever touches /Xft/Hinting so XFCE users will normally end up with XSETTINGS like this: $ dump_xsettings | grep Hint Xft/Hinting -1 Xft/HintStyle "hintslight" In GTK2 applications that seems to behave as expected: hinting will be enabled and use the "slight" style. However GTK3 will actually disable hinting in this case. (Is that a bug in GTK3? Maybe.) I noticed this because Firefox in Fedora 22 is now built against GTK3, and it was the only application in my environment which had messed up fonts. Eventually I figured out that under GNOME3, I get Xft/Hinting=1, but it is always -1 in XFCE.
I wrote a patch to also set /Xft/Hinting when setting /Xft/Hintstyle: https://github.com/danc86/xfce4-settings/commit/f9c9dda9dadf195536e2adb5c32bf138854c4e4b This works for me, after applying it I opened xfce4-appearance-settings and changed my hinting style and now I have /Xft/Hinting=1 in XSETTINGS and Firefox font rendering is fixed. But it has the downside that /Xft/Hinting will be left at -1 until the hinting style is changed. Ideally existing user's configs would be automatically updated to set /Xft/Hinting to a suitable value, but I don't know if/how that can be done.
Thanks for the bug report and patch. Verified and committed at: https://git.xfce.org/xfce/xfce4-settings/commit/?id=82a6f87bb7325f1a210ecaaa271fd2d6efbe1bbb