Downloaded the extension and did a test run.

First word searched: "Image".

[[ NO DEFINITION FOUND ]]

Second word searched: "Approved".

[[ NO DEFINITION FOUND ]]

Wow, fascinating.

"Recommended" by Mozilla, again proving that Mozilla would recommend trash or spyware extensions randomly, quality be damned. This extension was last updated 3 years ago, there is no reason to keep it as "recommended".

Works great, gives definitions as it's supposed to. Overall makes reading for a non-native person easier.

I just love this. Granted once in a while i come across a word it isn't familiar with, but that is rare. Also it pops up at times i'm right clicking for another reason, but that too is rare. It is simple to use and i love the pronunciation option to actually hear how the word is pronounced.

Works well, but has a thin vocabulary.

Fantastic! Excellent add-on.

It only shows "No definition found" now

What happened? It used to work mostly fine. It does not recognize any word now

Great addon. It saved much time for me.

It takes long to search

Need an option to DISABLE on certain URLS! Driving me nuts trying to edit text and double clicking text makes this pop up instead of edit the text box.

A very good one, with the exception of a few disadvantages like limited vocabulary
Edit: Doesn't work anymore. Such a shame. :(
Edit2: Thank you so much for the user who recommended 'definer' by lumetrium! Even though its not as simple as this one was, it does the job.