Occasionally the back button doesn't work. The issue appears to be intermittent. Example str: Visit https://reviewboard.mozilla.org/r/ Click the "Documentation" link under the "Support" menu. Attempt to go back. (This was reliably failing, but then I shift-refreshed and it started working again. It has also failed on other sites.)
I'm running e10s, by the way, so that might be related.
It is! :)