“Look up” is a macOS feature that can look up a word in the system Dictionary or Wikipedia. It can be activated by force-clicking or tapping with three fingers on a trackpad Mac, and by a right-click menu options in some apps (Firefox isn’t one of them) on all Macs. It is distinct from search.

The problem with force clicking is that the user doesn’t get to decide the string that is passed to the dictionary, instead a word boundary algorithm decides the word.

This algorithm doesn’t work very well on CJK-languages.

Oh I didn't know about these features on MacOS. Have never used it too much..