> Session would still store that last visit page's DocumentElement. However pages retrieved previously (even not matching current page url) would keep working.

No they would not. Any method call on the element using the driver under the hood (i.e. 99% of them) would be  broken as the driver would operate on the new page (Firefox has only one page open at a time. you cannot query the xpath in an older page without navigating back in the history first)