As part of project Async, we have been working on refactoring Firefox’ Session Restore to ensure that it does not block the main thread. Part of the work has been cleaning up the code and the data structures involved in Session Restore both to give us some maneuverability and to improve the chances of catching refactoring errors.

Unfortunately, a large number of add-ons seem to rely upon these undocumented data structures. Some of their features might therefore stop working. If you are the author of one such add-on, you should monitor carefully bug 874381 and its blockers. If you realize that we are about to break your add-on, please inform us asap, so that we can work out a solution.