Are you downloading pornography or a warez cracked program? Or are you dowloading an add on to help improve your firefox expierence? Theres a much greater chance that the pornography and warez software is going to contain a virus. Whats the file? If it is illegal or suspicious looking then its probably dangerous. Look over the site, if youre downloading a file from a basic website theres a higher chance that the site will have a virus thats hidden in there dowloadable file than from a site that looks like it's been made from years of dedicated web designers. Consider who you are dowloading the file from, think about it, if youre dowloading something from the windows its not likely youre downloading a virus. If people have said theyve dowloaded that file then its most likely safe for you to dowload.