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.