This drives me absolutely crazy as I tried to narrow it down for days now. The Amazon Seller website makes me login daily, which is not a problem on Chrome. If I export the cookies from Chrome into Firefox it works, but also forgets it after about day. I can confirm: - cookies are not the problem and don't expire, as they keep working in Chrome on the same machine and network and if I export them again from Chrome and import to Firefox even daily, it still works, so the cookies are fine - It's also not amazon's fault, let's say a stricter protection agains Firefox for example, for the same reason as above, they don't block the cookie - I turned off every protection, made sure cookies are persistent and I gave every exception to amazon's website but didn't help - it happens even if I don't close the browser, so it's not a problem with cookie deletion - I tried it in a completely clean new firefox profile, same problem I'm losing my mind because of this..