When you download Firefox from http://www.mozilla.com it will select one of the mirrors from around the world to download Firefox from, you could get a different mirror every time you try to download Firefox. Many universities allow Mozilla to use their servers to store Firefox. The one you mentioned is Aachen university in Germany, and it is one on the sites that Mozilla uses - http://www.mozilla.org/community/mirrors.html As long as you download Firefox using the download button at http://www.mozilla.com it will be a safe link to download Firefox from. As for the message about it could harm your computer, that is a standard message and applies to any executable file.