Indeed I just tried out Nightly and am disappointed it is missing. I have given them a little nudge so hopefully they add Look Up to the context menu before this hits stable.
I know this doesn't get you 100% of the way there, but pressing Cmd+Ctrl+D while hovering over a word brings up the "Lookup" window in Firefox 88 (similar to Safari).
I just tested this in Firefox and it works on the highlight which is inconsistent with how it works elsewhere. Usually it selects the word under the cursor not the selection. Perhaps Firefox is doing something to change the default behaviour?
