I added the favicon.ico file to the wp-content/themes/twentytwelve/ directory on the Apache server as documented on this site. I'm not getting the result, though. Firefox's Page Info shows no such media, and yes I tried clearing the browser cache.
Looking at the instructions further describing the header.php modification, I see that the twentytwelve/header.php does not contain such a line, and the View Source option in Firefox shows me that no such link tag was generated. (I recall in the past that browsers used to probe for it anyway; I guess that is no longer the case?)
So, is Twenty-Twelve not supporting favicon, or are we supposed to do it in some other way? Again let me point out that the default configuration is not matching what the instructions state.