Google announced that it will no longer be supporting its toolbar for FireFox 5 or future versions. In its blogspot post, Google stated that this was due to changes in the default setup on Firefox.

For Firefox users, many features that were once offered by Google Toolbar for Firefox are now already built right into the browser. Therefore, while Google Toolbar for Firefox works on versions up to and including Firefox 4 only, it will not be supported on Firefox 5 and future versions.
Of course, the big issue for us webmasters is that Firefox still does not have all the elements that are available in the Google toolbar, such as PR rankings. However, there are good alternative plugins available to fill this gap, like SEOQuake.