The apparent culprit is /etc/fonts/conf.d/10-hinting-slight.conf from the binary package fontconfig-config (source package fontconfig); Firefox uses the settings from this file instead of using the desktop's settings. Removing that file brings back the correct behavior (hinting matches desktop). I'm not sure whether this is a bug in Firefox or Fontconfig because other programs do not show this behavior.
The apparent culprit is /etc/fonts/ conf.d/ 10-hinting- slight. conf from the binary package fontconfig-config (source package fontconfig); Firefox uses the settings from this file instead of using the desktop's settings. Removing that file brings back the correct behavior (hinting matches desktop). I'm not sure whether this is a bug in Firefox or Fontconfig because other programs do not show this behavior.