https://support.mozilla.com/en-US/kb/websites+or+add-ons+incorrectly+report+incompatible+browser

You only mention downloading Firefox 4, just to confirm, did you run the setup file to install Firefox 4? You can also update to Firefox 4 by using the "Check for updates" option in the Help menu. If Firefox 4 has been installed, there may be a preference that is overriding the version details. # Type '''about:config''' into the location bar and press enter # Accept the warning message that appears, you will be taken to a list of preferences # Locate the preference '''general.useragent.override''', right-click on it and choose '''Reset''', then restart Firefox and it should identify itself as Firefox 4