Rated 5 out of 5 stars
Well, with firefox 57.0, this elegant and simple extension is now non-functional. I feel bad about constantly badgering the dev to upgrade the code when really it is Mozilla constantly dicking around that makes these things stop working. A pity, as clean, simple extensions like Dict are the most useful of all.
OLD REVIEW: I've used dict for years. It's the most straightforward and useful add-on of its type. But it apparently doesn't work with firefox 5.
EDIT: UPDATE - The problem isn't with Dict or with Firefox per se, it is with Adblock Plus. To avoid this, go to AdBlock preferences and check the filters in "EasyList" and uncheck the one labelled "data:text$popup".
Shout out to dev David Costanzo for figuring that out, he puts way more effort into support for his addon than many other developers.