Extensions for Firefox, such as the Google Toolbar, include a list of compatible Firefox versions. Currently, the Google Toolbar only goes up to Firefox 4. However, people have tested and it actually works on Firefox 5, so there are two workarounds:

(1) Edit a file to revise the range of compatible versions

(2) Install an add-on that lets you ignore the range of compatible versions

This thread has info on both approaches: google toolbar does not work with firefox 5.0. why not! | Firefox Support Forum | Firefox Help.

Any luck?

The Google Toolbar 7.1.20110512W version works in Firefox 5.0 by using the Compatibility Reporter extension, I tried it myself due to the large number of postings about GTB problems.
https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/