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.

It has become an integrated part of my browsing experience. English is my 2nd language. It is a strange experience to be without it.

It does not show up in my extension bar in Firefox. I had dig it out from withing the extension-menu. Took me a while to remember where this awesome add-on originates from.

smooth & fast! i love the text to speech voice pronouncer! can i know her name!? so i can use her with my other text to speech programs!

if you could just please add some custom searches to it, and also please more than one language at the time (see definer ext) , this would be perfect, definer is good but its HUGE for almost same thing, I love tiny extensions that are actually useful like this one. please also add ability to disable bubble on some sites or make it smaller or appear on a fixed location (like following mouse cursor so you can take the bubble out of the way of the text you are reading) use some other way to show result.

Can be perfect but aint :) LeftDoubleMClick - is OK - but ...maybe RightButtonDMC will be better, or any choice? LDMC collides with ...many! And what with links?

I keep downloading this ext because I love the prospect and I keep running in to the same issue once I have it downloaded where no matter what I do the definition bubble that pops up will not close at all. No matter what I do. Then I delete the ext, a few months go by, I go to the ext shop to look for useful things, download it, and start the whole process over again.

Excelente extension, muy util. Como comentario creo que haria falta soporte para google docs

Holy shoot this is amazing

excelent

So many words are missing. Mostly useless.

This used to work great, but now ever word is searched for and not found. Rendering this completely useless.