If you've installed Firefox 5 and noticed that
Google Toolbar wasn't updated to support the new Firefox release, there's a simple way to enable the extension:
install the
Add-on Compatibility Reporter.
"After installing the Add-on Compatibility Reporter, your incompatible
extensions will become enabled for you to test whether they still work
with the version of Firefox or Thunderbird that you're using." This
should only be a temporary solution until Google Toolbar and other
extensions update their compatibility list.

Google Toolbar 7 works well in Firefox 5, especially considering that the new Firefox version
didn't make too many important extension-related changes.
Firefox's faster release cycle,
inspired by Google Chrome, has an important downside: extension
developers need to update their extensions more frequently and update
the list of Firefox versions that are supported. Mozilla alleviated this
problem by automatically marking almost 4,000 extensions as
compatible with Firefox 5, but Google Toolbar is not hosted by Mozilla and it's downloaded from Google's servers.