Ask HN: How does Facebook know what I buy on Amazon or vice versa?
I have had similar experiences. Others have already mentioned some cross-browser fingerprintig techniques. One of the worst that many people don't know about is that browsers hand over your local IP. Check this proof of concept:<p><a href="http://net.ipcalf.com/" rel="nofollow">http://net.ipcalf.com/</a><p>The media device IDs the browsers provide look even worse:<p><a href="https://jsfiddle.net/u4n4s296/" rel="nofollow">https://jsfiddle.net/u4n4s296/</a><p>I am not sure if these are unique to the device type (for example a certain soundcard model) or to the device itself. If it's the latter, then that is an indestructible cross-browser cookie right there. EDIT: As per icebraining's comment, in Firefox they are not not cross-domain, not cross-browser and get randomized when you delete your cookies.