1. Go to: http://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0/win32/

2. Click on the folder that matches the language you would like (en-US is united states English)

3. Download Firefox Setup 4.0.exe and install it in place of the newer version.

Google needs to update the Google Toolbar for Firefox 5. Firefox 4 is no longer supported by Mozilla and shouldn't be used, Firefox 5 was a security update.

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/

What was your previous version, and did you have your bookmarks backed up? I had a similar problem, upgrade from FF 3.6.18 to 5. Lost all my bookmarks, and couldn't restore them from my back-up. If you go to [ftp://ftp.mozilla.org/pub/mozilla.org.../releases/] you can select any version for download and installation. Be aware that FF is not supporting any release below 3.6. If you have your bookmarks backed up, or in a folder in C:\Documents and settings\user\application data\Mozilla\firefox\profiles\h3rkn358.default\bookmarkbackups ( The number on your .default folder may be different ) you can select bookmarks\organize bookmarks\backup or restore, from your FF toolbar and navigate t the back-up file and open it. Good to go.