Various small fixes

Some suggestions of mine:
All emails should be stored in lower case. Furthermore, I remove the "Skipping server" debug output as it produces far too many messages as getIdentities is called fairly often.

I think the null check in misc.js should be unnecessary by now, so I removed that, too.
It's a little bit unorthodox to change the properties of an XPCOM object like this, and the user might actually want to see John.Doe@example.com in the outgoing mails... I don't think that's a big issue, though. What do you think?