To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data.

You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

What problems are you having with Firefox 4?

There are other things that need your attention.

Your above posted system details show outdated plugin(s) with known security and stability risks that you should update.

1. Shockwave Flash 10.0 r32
2. Next Generation Java Plug-in 1.6.0_23 for Mozilla browsers

Update the Flash plugin to the latest version.

• http://kb.mozillazine.org/Flash
• http://www.adobe.com/software/flash/about/

Update the Java plugin to the latest version.

• http://kb.mozillazine.org/Java
• http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform: Download JRE)