You can use this extension to force compatibility of the Google Toolbar extension. It seems to work fine for me in 5.0, but if you do find a problem with the GTB extension, you can report problems thru the "Compatibility" button in the Add-ons Manager tab.
https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/

Here is the link to download the last version of 4 If you are in the US http://mozilla.osuosl.org/pub/mozilla.org/firefox/releases/latest-4.0/win32/en-US/

Thank you!! Great help.

Keep using Firefox 5, it has better security. Google Toolbar still works just fine with Firefox 5, but until Google releases a toolbar compatible with Firefox 5, you need to disable add-on compatible versions verification. You can do this by installing Mozilla's Add-On Compatibility Reporter extension from https://addons.mozilla.org/en-US/firefox/addon/15003 and restarting Firefox twice. It basically tells Firefox to ignore the maximum compatible version so the toolbar can be enabled on version 5 too, not blocked just because Firefox releases major version numbers like mad just for the sake of catching up with IE9 and Chrome 12.