You receive two cookies on the first load of the page. Since The server cannot read if the cookies were accepted until the page is reloaded, that's why there's a reload button. If you reload, then on the lines below, a description of both cookies should appear:
Then, pressing any of the other buttons, you should see either that the invisible label becomes visible with info similar to that one provided by opera, or that it appears on a popup. I don't have *any* experience with firefox, so I can't tell, although I've DLed it, I'm not that eager to install more soft on this machine, so I'll probably start up the linux box and test it there.
The thing is, if you're using MSIE6.01 or superior, the cookies that include HttpOnly cannot be read from a script within the page, thus making them in theory invulnerable to xss cookie hijacking. I say in theory, cause you could create a java applet that accessed directly the page and stole the whole http headers, but then the site must already be xss vulnerable, as usual. Somebody mentioned a combination of flash, java and a script could in theory allow you to steal the cookie, so this new behavior of the navigator is not a panacea, but it's a first step, which makes it much harder to properly abuse an xss vulnerability. But not impossible. Navigators that don't support the httponly tag, should degrade gracefully and have no further problems (but no special immunity also) whatsoever.
Fiu, whether it works or not, you can't say I didn't write about it