"For the sessionStorage object that's used to store data that persists only through the lifetime of the browser session, it differs from JavaScript cookies in that the data does not persist across browser tabs. The data does survive a browser crash in some browsers, however, such as FF."
-- http://www.javascriptkit.com/javatutors/domstorage.shtml#.UkEKx2RsNhY (not a pretty website, but explains it the most succinctly)