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-fin...

Any docs or more info on this? Its not working for me.