Translate any word at any time. The Wiktionary and Google Translate add-on for Firefox will give you an in-line dictionary for any word you point to or double-click; whole page translation of any web page in another language.
Here is a little more about the add-on from the developer:
Move your mouse over the word that you want to translate then press down the Ctrl+Shift keys (or Ctrl, or Ctrl-Alt — configurable with the options dialog) to show the meanings of the word.
Alternatively, with a little change on the option, you can double-click a word or a phrase to show the definition of it in your language. At the bottom of the little pop-up, there is a “more” link which will lead you to the full definition at Wiktionary web site. To close the little pop-up, just click on any blank place on the browser (or move the mouse away — set with the options dialog). The little pop-up must be closed before you can activate another in-line translation.
Pick up this add-on at the Firefox Add-ons website.