Digital Wanderer

Yesterday I was looking for a Firefox extension like this: you hover your mouse over an English word, and a Russian translation pops up.
Found something similar, but without Russian - Oak Vocabulary.
After struggling a bit, I added Lingvo to it.

Want it for yourself? Here's how you can add Russian.
Install the extension, right-click on the little blue book at the bottom, select Configure Dictionaries, hit the Add button there and fill in the fields:

Dictionary: Lingvo
Search URL: https://lingvo.yandex.ru/q?text=$$$$
charset: utf8
Parser: ret = getPartHtml(html, '<DIV LANG=ru', '<div', '</div>', 'No entry found for');

Then hit OK and it's ready to use:
either double-click a word on the page, or right-click and Check, or hover your mouse over a word while holding Control.

As a bonus: the dictionary translates not just from English, but from other languages too.