This was brought up in https://github.com/mozilla/browser.html/issues/372 that events received don't carry enough information to know if they are related to a current session or previous session. Unfortunately event.detail.url is not enough to make for that either, see quoted comment below: > I think it's helpful but not a complete solution for the problem, as it won't cover following scenario: > iframe.src = foo > iframe.src = bar > iframe.src = foo > When you get first event.detail.url it will be equal to iframe.src but it won't > actually mean that document was loaded. > I think iframe and event.detail would have to carry some other field maybe sessionID or that user would be able to assign to an iframe like: iframe.src = url; iframe.sessionID = ++guid; including sessionID along with an events like event.detail.sessionID would allow an event handler to assess if received event belongs to a current session - page load or not. > While this is unrelated same iframe.sessionID ideally would also be used to deal with a cache so instead of doing iframe.goBack() one would be able to say iframe.restore(sessionID). > Maybe iframe.sessionID can be generated automatically if user does not sets it or maybe it's just generated all the time, but I think such a fix would make a lot more sense.

Also see: https://github.com/mozilla/browser.html/issues/382

Firefox OS is not being worked on