Firebug "double post" with Ajax, and cookies

Over the past few months I've learned quite a bit about the complexities of web page security. I've gotten to be fairly decent, despite lacking a lot of basic knowledge - but even though I've learned a lot, there's still some questions I can't quite figure out.

First of all, I'm using Firebug to submit an Ajax request. I get the response:

Firebug needs to POST to the server to get this information for url: http://www.example.com/helloworld/test

This second POST can interfere with some sites. If you want to send the POST again, open a new tab in Firefox, use URL 'about:config', set boolean value 'extensions.firebug.allowDoublePost' to true This value is reset every time you restart Firefox This problem will disappear when https://bugzilla.mozilla.org/show_bug.cgi?id=430155 is shipped.

Analyzing the script suggests that this is because the data that I POST is then being POSTed again. But what I don't quite understand is, why does this interfere with the Ajax request?

Secondly, I was under the impression that pretty much any non-secure server that uses cookies is vulnerable to session hijacking simply by copying another user's cookies. But then I've recently learned that this isn't quite true - there's another factor. I suppose there's the browser, the IP address, the referrer that could all be factors… what else?