@matachi, I think I had the same problem with not being able to see the Desktop Integration in the Preferences window of firefox. But, after I restarted Firefox it showed up. This might have happened if I did my first Software Update after a fresh install between the time I started the session that did not show the option and the time I started a new session because the webapp packages nor the "technical items", which they are members of, show up in Ubuntu Software Center until you do the first Software Update after installing Ubuntu 12.10, at least for fresh installs.
@matachi, I think I had the same problem with not being able to see the Desktop Integration in the Preferences window of firefox. But, after I restarted Firefox it showed up. This might have happened if I did my first Software Update after a fresh install between the time I started the session that did not show the option and the time I started a new session because the webapp packages nor the "technical items", which they are members of, show up in Ubuntu Software Center until you do the first Software Update after installing Ubuntu 12.10, at least for fresh installs.