You can install Firefox 3.6.* available from http://www.mozilla.com/en-US/firefox/all-older.html 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 You should contact Kaspersky about their plans to support Firefox 4. the main application should work just fine with Fireofx 4, it will be any add-ons that may not be working.

Thanks Tony. I will try!