Translate selected word or phrase over translate.yandex.ru from any lang to Russian (or from Russian to English) - result in context menu (right click) and in popup. Also you can select text and push the button or Ctrl+Shift+T and translated text will replace selected text. You can change hotkey in preferences.

Minimum and fast. Unofficial extension - works using official API.

https://addons.mozilla.org/en-US/firefox/addon/yandex-translate-in-context/