How reliable is cross browser detection?

Unfortunatelly not 100% reliable. Some recent browsers use their own dedicated storage area for Flash Cookies and thus the cookie won't be seen with other browsers. Also the "Stealth Mode" oder "Incognito Mode" in modern browsers prevents any cross browser detection. Also iOS mobile devices cannot run Flash and therefore Flash cookies cannot be saved. But we also feature several other (eg. HTML5) methods to detect multiple accounts. And of course all regular HTTP cookies are fully cross browser compatible and this add-on makes use of them.