Gecko removed it in https://bugzilla.mozilla.org/show_bug.cgi?id=687400 and it will soon be expunged from DOM4 as per: http://lists.w3.org/Archives/Public/public-webapps/2011JulSep/1361.html
As mentioned in the Gecko bug, there is quite a number of uses found by code search. And there is already a report of site brokenness. It's clear that this method causes confusion, but it's quite harmless (minor performance degradation when compared to using ===).
Firefox added it back here - https://bugzilla.mozilla.org/show_bug.cgi?id=1256299 We have WPT coverage as well - https://wpt.fyi/results/dom/nodes/Node-isSameNode.html?label=master&label=experimental&aligned&view=subtest&q=issamenode I think this bug can be marked as "RESOLVED INVALID" or "RESOLVED WONTFIX". Thanks!