If no one has found a fix for the session startup bug, I think I might have.
In browser/components/sessionstore/src/nsSessionStore.js:306 - change to this._prefBranch.getIntPref("sessionstore.max_concurrent_tabs") == 0; (was "sessionstore.restore_on_demand",
which was changed in browser/app/profiles/firefox.js). In browser/components/sessionstore/src/nsSessionStartup.js:157 & 158 - comment these lines out (telemetry was removed from the package-manifest.in file).
In toolkit/components/url-classifier/content/listmanager.js:111 & 112 - comment these lines out (HashCompleter was removed from the package-manifest.in file).
(It no longer recognizes telemetry or hashcompleter as proper services).
I might be wrong, however, I have restored a session of mine from a few weeks ago. So for me, this worked.
I apologize for my formatting of these fixes. I hope I have been of some help.