There are two possible values for X-Frame-Options:

DENY

The page cannot be displayed in a frame, regardless of the site attempting to do so.

SAMEORIGIN

The page can only be displayed in a frame on the same origin as the page itself.

In other words, if you specify DENY, not only will attempts to load the page in a frame fail when loaded from other sites, attempts to do so will fail when loaded from the same site. On the other hand, if you specify SAMEORIGIN, you can still use the page in a frame as long as the site including it in a frame it is the same as the one serving the page.

To configure Apache to send the X-Frame-Options header for all pages, add this to your site's configuration:

Header always append X-Frame-Options SAMEORIGIN

When an attempt is made to load content into a frame, and permission is denied by the X-Frame-Options header, Firefox currently renders about:blank into the frame. At some point, an error message of some kind will be displayed in the frame instead.