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the extension is installed, but how to download the dictionary ? any link please?

Это отзыв для предыдущей версии этого дополнения (2.59). 

Hello!

If the extension is installed, the extension is the dictionary.

You just need to turn it on:
http://marcoagpinto.cidadevirtual.pt/en_installing.html

See points:
3.1 & 3.2

And remember that to use it in Thunderbird, you will need to install it there.

To use it in Firefox, you need to install it there.

(to use on both, you need to install on both).