Hi ES74,
My recommedation is that you need to use a lower version of Firefox 25.0, such as 24 or lower. a recommendation is to uninstall Mozilla Firefox on that machine and reinstall LoadRunner 12.
It will set up the version 22.00 to be tested, I attached an screenshot that displays the location and the details about the Mozilla Firefox.
Then try again and let us know.
Kind Regards,