I found it! It's the "Look up & data detectors" setting under System Preferences > Trackpad. For whatever reason mine was set to "Force Click with one finger" w

I found it! It's the "Look up & data detectors" setting under System Preferences > Trackpad. For whatever reason mine was set to "Force Click with one finger" which I never use and had trouble consistently activating. The other option is "Tap with three fingers." ⌘-^-D the keyboard shortcut also works. Firefox has had some bugs around this. https://superuser.com/questions/1138014/how-to-use-three-finger-tap-for-dictionary-lookup-in-firefox https://superuser.com/questions/1138014/how-to-use-three-fin...