In Firefox and other similar browsers there may be a security warning displayed every time a search is performed. While this is annoying, it is safe to click 'Ok' and follow the redirect. Your browser is simply redirecting the search to Startpage.com.
If you are using Firefox and would like to avoid seeing this message every time you do a search you can simple toggle network.http.prompt-temp-redirect in about:config to false.
Please note: As of Firefox 40(?), this is now the default and the above step shouldn't be needed anymore.