Side note, this already doesn't work if you don't have a Touchpad / Magic mouse. The normal workflow is Right Click ==> Look Up, but Firefox overrides macOS's normal right click menu. :(
Firfox is just not a very good macOS citizen, sadly.
Oh, there is that, but it's too far away to be useful.
If I could set up a custom keyboard shortcut for the service, it would work great—but support for custom keyboard shortcuts also doesn't work in macOS Firefox[1]! (Unless you set a combination which Firefox doesn't already use for something, and it already uses everything sensible, so you have to set something long and annoying.)
Firfox is just not a very good macOS citizen, sadly.