Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
This offers examples of sending POST data to the server and displaying the server response.
Need more elaborate examples, examples of displaying the response in a new tab, in background tabs, and a link to using XMLHttpRequest for POST requests.
Preprocessing POST data
The aPostData argument of the (global) loadURI(), openDialog(), and (tab)browser.loadURIWithFlags() methods expects the POST data in the form of an nsIInputStream (because they eventually call nsIWebNavigation.loadURI()) while POST data can be created using nsIMIMEInputStream. Most of the time, POST data starts as a data string in the form of "name1=data1&name2=data2&...", so you must convert it before passing the data to one of the methods. Here is an example:
var dataString = "name1=data1&name2=data2";
// POST method requests must wrap the encoded text in a MIME
// stream
const Cc = Components.classes;
const Ci = Components.interfaces;
var stringStream = Cc["@mozilla.org/io/string-input-stream;1"].
createInstance(Ci.nsIStringInputStream);
if ("data" in stringStream) // Gecko 1.9 or newer
stringStream.data = dataString;
else // 1.8 or older
stringStream.setData(dataString, dataString.length);
var postData = Cc["@mozilla.org/network/mime-input-stream;1"].
createInstance(Ci.nsIMIMEInputStream);
postData.addHeader("Content-Type", "application/x-www-form-urlencoded");
postData.addContentLength = true;
postData.setData(stringStream);
// postData is ready to be used as aPostData argument
...
POSTing data to the current tab
There is a convenience method in global scope (in Firefox, chrome://browser/content/browser.js):
loadURI(aURI, aReferrer, aPostData, aAllowThirdPartyFixup);
POSTing data to a new window
window.openDialog('chrome://browser/content', '_blank', 'all,dialog=no',
aURI, aFlags, aReferrer, aPostData);