It works, but on every page I visit, it says that a script is blocking the direct opening, it shows "open PUD", and only when clicking it, it opens up the popup with the definition. So, no problem, it works, but there's probably something that bugs a little, as there's this warning message.

The brilliant point is that we can add custom dictionaries!

Works great, but sometimes picking a different dictionary doesn't take effect until I reload the tab

It's very wonderful add-on for me. But, I can't use usual selecting and right-clicking with this add-on. I want to use "Context Search" (which make it possible to search selected text from context menu) and this add-on together. I wish to become it possible.

This works for English definitions but not by default.
Go to settings.
1. Only check Enable Pop-Up Dictionary or you might not get the window.
2. Replace default dictionary with:
http://www.wordreference.com/definition/$$
as the standard one translates to French.
3. Double-click or highlight a word to get your definition.
It becomes harder using other right-click functionality, but you can manage.

When I double click a word a box with English to French translation comes to screen and I do not know French, unfortunately. Can the dictionary be changed someway, that is do I have to first upload a new dictionary and how? I would like to see a unknown word defined in English, this is usually enough to grasp about the word's meaning.
The options page doesn't offer any solution.
Earlier a word's definition could be found with just a double click, perhaps together with alt or cntrl key, and a box with synonyms appeared on the screen. Or is this not permitted with the latest Firefox version? Babylon and Dictionary.com with this kind operation were very useful add-ons and are missed greatly now.
Otherwise Pop-up dictionary has many advantages over other similar add-ons. Thanks very much !
Matti Ollikainen