Comment 51 for bug 956618

For non-LDAP cases, I understand that it's this addToResult(...) function which finally sorts the autocomplete results list:

http://mxr.mozilla.org/comm-central/source/mailnews/addrbook/src/nsAbAutoCompleteSearch.js#276

Seems to first sort on popularity here (after having removed duplicates)...
http://mxr.mozilla.org/comm-central/source/mailnews/addrbook/src/nsAbAutoCompleteSearch.js#309

...and then on full addresses here (probably secondary sort):
http://mxr.mozilla.org/comm-central/source/mailnews/addrbook/src/nsAbAutoCompleteSearch.js#318