[jeremiahlee]

If you hover the cursor over the word and hit command control d, you’ll get the definition in Firefox.

[dkonofalski]

That's helpful. Thanks. Would still like for it to be in the context menu, though, but this will do in a pinch.

[jhatax]

Another trick if you have the new "Magic" trackpads: Select the word, place three fingers on the pad, press down (force press), and you can see the definition of the selected word (will be highlighted in yellow).

[mweibel]

Only one finger needed