Better to stick with current version of Firefox. Firefox 4 is end of life, it is not supported. '''see''' https://support.mozilla.com/en-US/questions/841081 for how to use the Google Toolbar with FF5 with support from neither Google nor Mozilla. Unfortunately it has consequences of not being able to use Microsoft product email applications properly. As seen in a newsgroup without a reference a Google document. Which might explain why they are working on an Internet Explorer solution first. Or continue reading Q841081 for alternatives, or a Google supplied list of alternatives at the very bottom.