It's a problem with your server.
Those characters are usually hidden: they mark the file as being encoded with UTF-8 (unicode). All files and text used by Construct 2 are encoded in UTF-8 to ensure it works in all languages across the world.
However, your server is set up differently and appears to try to force the ISO-8859-1 character set instead of UTF-8. I guess your server is set up to serve Portuguese text, and this conflicts with the UTF-8 text in the HTML. Note that only Firefox gets confused and displays the UTF-8 marker: Internet Explorer and Chrome seem to still figure out that they should use UTF-8. Using a tool to check server headers, you can see your server reponds with
Content-Type: text/html; charset=ISO-8859-1
Scirra.com responds with:
Content-Type: text/html; charset=utf-8
Your server should really be using UTF-8 since it's the only way to ensure that text is displayed properly for everyone regardless of their language. If you don't know how to do it get in touch with whoever runs your server. Another workaround which isn't as good is to simply convert your index.html file to the ISO-8859-1 character set (a program like Notepad++ is good for this), but it will keep happening with files Construct 2 exports until you fix the server.