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/

ImTranslator includes the text-to-speech functionality that converts text into voice.
The text-to-speech conversion is available for 30 languages. You can listen to the original text or its translation.
http://about.imtranslator.net/tutorials/presentations/imtranslator-translator-for-firefox/text-to-speech-imtranslator/

Using Never remember history is equivalent to always being in Firefox Private Browsing mode.
Privacy Options (click the Firefox menu, choose Options and select the Privacy & Security panel).
For more information, see:
https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history