HEADS-UP: Mozilla has disabled new versions of this extension as per June 1st, 2019 which has broken it. The dev is working on a fix but this is not his fault so kindly stop review bombing this. To users looking to install: no it doesn't work right now but it will.

Source: https://github.com/krv5vignesh/Dictionary-extension/issues/32

EDIT: It just werks on my machine again as of 23/06/2019. 5 STARS