Firefox 4.0/4.0.1 isn't supported, 5.0 was a security update for 4.0.1.

Google needs to update the Google Toolbar program for Firefox 5.0.

The Google Toolbar 7.1.20110512W version does work pretty good in Firefox 5.0 by using the Compatibility Reporter extension. But not every feature works 100%.
https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/

http://googlesystem.blogspot.com/2011/06/enable-google-toolbar-in-firefox-5.html

Thanks very much for this information. I'll have to give it a try on my desktop PC. Unfortunately I'm leaving town for a long weekend and won't be able to try it until I return. Meanwhile, someone pointed me to a download location for 4.0.1, and I was successful in dropping back to that release: http://www.oldapps.com/firefox.php?old_firefox=114 In general, it looks like a nice site to keep in mind: http://www.oldapps.com/