One of the possible causes of bug 1455828 is a bug somewhere causing unrelated data to be written to the shared memory file. Currently anything written would be appended and ignored by the pref deserializer, but with the patch in bug 1447867 it would write to the beginning and cause the kind of parse error we saw. That change could be made independently of the rest of the patch; if we do that and start seeing prefs parser crashes on Nightly again, we'll know that that's the problem. I'd like to have bug 1456902 taken care of before trying that, so that if that does happen, the crash reports can maybe give us a hint about the root cause.

This wound up not being needed; relanding bug 1447867 as-is (after fixing some suspected causes) worked.