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So I thought I have found a bug of Firefox, which treats sessionStorage differently than Google. I found on Firefox that when the page was directed to another url, the sessionStorage was freed immediately. At first I thought it was a bug of Firefox. To get around of it, I use localStorage instead, and send the author a pull request. But the original author reminded me that he hadn't found such problem on Firefox. Than it occurred to me that it might be my problem. So I looked into my code again, did find the problem. It turns out I am using sessionStorage inside an iframe, when change the url of the top window, then on Firefox the storage is freed immediately as the session is lost.