https://www.mozilla.org/en-US/mailnews/specs

[tanstaafl]

Does anybody know where the specs at https://www.mozilla.org/en-US/mailnews/specs/ were moved to?

It isn't http://www-archive.mozilla.org/mailnews/specs/ . For example, that doesn't have the message filter spec that stated you can create any number of custom headers and message filters. I found https://wiki.mozilla.org/Filters , but that is not the specification I had linked to in http://kb.mozillazine.org/Limits_-_Thunderbird . I found https://wiki.mozilla.org/Mork but that doesn't have the same information about Mork that used to be in mailnews/specs .

The wayback machine doesn't have that web page. All of its copies either have it replaced by a different web page or report crawler errors.

[wsmwk]

http://www-archive.mozilla.org/mailnews/specs/ seems to be fairly complete to me.
and it has a link to http://www-archive.mozilla.org/mailnews/specs/filters/ which has mentions custom headers