The issue has been fixed. The new update 15.54 just has been published.
Sorry for the inconvenience.

For your reference, the links to user guide are:
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/