nsISessionStore
ong aindex); nsidomnode forgetclosedwindow(in unsigned long aindex); astring getbrowserstate(); unsigned long getclosedtabcount(in nsidomwindow awindow); astring getclosedtabdata(in nsidomwindow awindow); unsigned long getclosedwindowcount(); astring getclosedwindowdata(); astring gettabstate(in nsidomnode atab); astring gettabvalue(in nsidomnode atab, in astring akey); astring getwindowstate(in nsidomwindow awindow); astring getwindowvalue(in nsidomwindow awindow, in astring akey); void init(in nsidomwindow awindow); void persisttabattribute(in astring aname); void restorelastsession(); void setbrowserstate(in astring astate); void settabstate(in nsidomnode ata...
... gettabvalue() returns the value corresponding to a key on a given window.
... astring gettabvalue( in nsidomnode atab, in astring akey ); parameters atab the tab on which to get the value.

Session store API - Archive of obsolete content
in order to properly restore your extension's state when a tab is restored, it needs to use the session store api's settabvalue() method to save any data it will need in order to restore its state, and then call gettabvalue() to retrieve the previous setting when the tab is restored.
... fetching a saved value you can fetch a value associated with a tab at any time (whether the tab is in the process of being restored or not), using code similar to the following: var ss = components.classes["@mozilla.org/browser/sessionstore;1"] .getservice(components.interfaces.nsisessionstore); var currenttab = gbrowser.selectedtab; var retrieveddata = ss.gettabvalue(currenttab, "key-name-here"); after this code executes, the variable retrieveddata contains the value saved for the key "key-name-here".