If you just upgrade to Firefox 5 and your Google bar is not working here is a trick.
Go to C:\Users\[your user name here]\AppData\Roaming\Mozilla\Firefox\Profiles
Search for install.rdf(there is one for every add-on you have).
Open up the install.rdf with notepad and change maxVersion from 4.0.* to 6.0.*.
Restart Firefox or re-install Google Bar and it should appear again.
