To translate with a shortcut key hold the Alt(default) key BEFORE or AFTER selecting the text with the mouse. The Bubble Translator will pop-up with the translation.
You can set any permitted shortcut key using the Pop-up Bubble Options menu.
https://about.imtranslator.net/tutorials/presentations/imtranslator-translator-for-firefox/bubble-translator-options-firefox/
For all technical issues please use the Feedback form in ImTranslator Options