For your reference:
User guide: http://about.imtranslator.net/tutorials/presentations/imtranslator-translator-for-firefox/
How-to: http://about.imtranslator.net/how-to/how-to-use-imtranslator-extensions/
FAQ: http://about.imtranslator.net/faq/faq-imtranslator-for-firefox/

How to translate on double-click? :
http://about.imtranslator.net/how-to/how-to-use-imtranslator-extensions/translate-on-double-click/

How to auto translate on mouse selection?:
http://about.imtranslator.net/how-to/how-to-use-imtranslator-extensions/how-to-translate-on-mouse-selection/

Run ImTranslator with Shortcuts:
http://about.imtranslator.net/faq/run-imtranslator-with-shortcuts/