I stopped using Firefox sometime back when I accidentally upgraded to version 5.0 form 4.0.1 and regretted for several reasons. It was a minor upgrade from earlier version but it broke many add-ons as they were not compatible with many add-ons and Google Toolbar is also not available from Firefox ver 4 onward.
So after waiting for a while, I finally decided to downgrade Firefox after Google announced it will not provide toolbar for Firefox 5 or higher versions. if you want to follow suit, here is how you can downgrade from Firefox 5 to Firefox 4 without losing your settings, bookmarks, favorite add-ons:
Downgrading from Firefox 5 to Firefox 4.0.1 or older versions
1. Open ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/ in your browser.
2. Browse Firefox 4.0.1 folder > Win32 folder > en-US
3. Download and install Firefox setup 4.0.1 exe file
Once you Firefox is downgraded to version 4.x., you can download and install Google Toolbar for Firefox
Hope that helps.