With this add-on you can translate any text to your own language with one click or hot-key. You can either translate the selected text, this way the translated text will overwrite the original selected text, or you can translate the full page too.
Here is a little more information about Google Translator from the developers:
It’s highly recommended to select less text than more, otherwise the translation would appear slower. You can select a maximum of 38000-character long text, if you select longer, you’ll get an alert box in some cases. You shouldn’t either select pictures and tables because these will disappear and mystic text will appear there.
You can pick up the Google Translator for Firefox from the Firefox Add-ons website.