I got a 403 from using the Firefox searchbar. I got a normal result from the webpage. I had FasterFox installed, but uninstalling it had no effect. Uninstalling various things had no effect so I nuked the lot – every damn thing – including the registry entries. A reinstall has cured the problem – but it’s a major PITA redoing a browser. All the saved passwords, logins, plugins, plugin settings – serious things are browsers.