Ensure Landmarks runs correctly when navigating the history

[matatk]

This seems to work fine on Chrome-like browsers (as others have noted in the bug report linked below). However, on Firefox, sometimes when going back/forward, the content script doesn't bootstrap again. This is because the page was only hidden, not destroyed and re-loaded, so the content script was not re-injected. Therefore the method for detecting that situation, proposed in the bug report, should be adopted.

[matatk]

I've tried this with both the in-devleopment 2.4.0 code and the released 2.3.1 code and get the same problem, so the cause is not down to the move from message-passing to port-based communications. It seems to be related to Firefox's "back-forward cache" ("bfcache") and I have added some info to some Bugzilla bugs, but not seen much movement on them, though there were some helpful suggestions from the bugs' reporters.

Ideally I would have the time and expertise to really get to the bottom of it (thanks @graemecoleman for the suggestion of actually modifying ExtensionParent.jsm to add debugging info) and maybe file a new bug specifically for whatever the root problem is. However, at the moment there are other Landmarks features (and other projects) I'd like to work on (and would likely be better at), so will leave it for now.

This only happens when going back in the history a certain number of steps, and can be mitigated by reloading (though Landmarks doesn't tell the user that), so it doesn't seem like a showstopper, though hopefully a solution will come to light soon.