Downloading Files

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.

Downloading files

To download a file, create an instance of nsIWebBrowserPersist and call its nsIWebBrowserPersist.saveURI() method, passing it a URL to download and an nsIFile instance representing the local file name/path. 

Components.utils.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
const WebBrowserPersist = Components.Constructor("@mozilla.org/embedding/browser/nsWebBrowserPersist;1",
                                                 "nsIWebBrowserPersist");
var persist = WebBrowserPersist();
var targetFile = Services.dirsvc.get("Desk", Ci.nsIFile);
targetFile.append("file.bin");
// Obtain the privacy context of the browser window that the URL
// we are downloading comes from. If, and only if, the URL is not
// related to a window, null should be used instead.
var privacy = PrivateBrowsingUtils.privacyContextFromWindow(urlSourceWindow);
persist.persistFlags = persist.PERSIST_FLAGS_FROM_CACHE
                     | persist.PERSIST_FLAGS_REPLACE_EXISTING_FILES;
persist.saveURI(uriToSave, null, null, null, "", targetFile, privacy);

If you don't need detailed progress information, you might be happier with nsIDownloader.

Downloading Binary Files with a Progress Listener

To download a binary file with custom progress listener:

Components.utils.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
const WebBrowserPersist = Components.Constructor("@mozilla.org/embedding/browser/nsWebBrowserPersist;1",
                                                 "nsIWebBrowserPersist");
var persist = WebBrowserPersist();
var targetFile = Services.dirsvc.get("Desk", Ci.nsIFile);
targetFile.append("file.bin");
var obj_URI = Services.io.newURI(aURLToDownload, null, null);
// Obtain the privacy context of the browser window that the URL
// we are downloading comes from. If, and only if, the URL is not
// related to a window, null should be used instead.
var privacy = PrivateBrowsingUtils.privacyContextFromWindow(aURLSourceWindow);
var progressElement = document.getElementById("progress_element");
persist.progressListener = {
  onProgressChange: function(aWebProgress, aRequest, aCurSelfProgress, aMaxSelfProgress, aCurTotalProgress, aMaxTotalProgress) {
    var percentComplete = Math.round((aCurTotalProgress / aMaxTotalProgress) * 100);
    progressElement.textContent = percentComplete +"%";
  },
  onStateChange: function(aWebProgress, aRequest, aStateFlags, aStatus) {
    // do something
  }
}
persist.saveURI(obj_URI, null, null, null, "", targetFile, privacy);

Downloading files that require credentials

Before calling nsIWebBrowserPersist.saveURI(), you need to set the progressListener property of the nsIWebBrowserPersist instance to an object that implements nsIAuthPrompt. Normally, nsIAuthPrompt expects a prompt to be displayed so the user can enter credentials, but you can return a username and password credentials directly without prompting the user. If you want to open a login prompt, you can use the default prompt by calling the window watcher's getNewAuthPrompter() method.

Components.utils.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
const WebBrowserPersist = Components.Constructor("@mozilla.org/embedding/browser/nsWebBrowserPersist;1",
                                                 "nsIWebBrowserPersist");
var persist = WebBrowserPersist();
// Obtain the privacy context of the browser window that the URL
// we are downloading comes from. If, and only if, the URL is not
// related to a window, null should be used instead.
var privacy = PrivateBrowsingUtils.privacyContextFromWindow(urlSourceWindow);
var hardCodedUserName = "ericjung";
var hardCodedPassword = "foobar";
persist.progressListener = {
  QueryInterface: XPCOMUtils.generateQI(["nsIAuthPrompt"]),
  // implements nsIAuthPrompt
  prompt: function(dialogTitle, text, passwordRealm, savePassword, defaultText, result) {
    result.value = hardCodedPassword;
    return true;
  },
  promptPassword: function(dialogTitle, text, passwordRealm, savePassword, pwd) {
    pwd.value = hardCodedPassword;
    return true;
  },
  promptUsernameAndPassword: function(dialogTitle, text, passwordRealm, savePassword, user, pwd) {
    user.value = hardCodedUserName;
    pwd.value = hardCodedPassword;    
    return true;
  }
};
persist.saveURI(urlToSave, null, null, null, "", nsFileInstance, privacy);

The above is going to give you errors about missing nsIDownloadProgressListener methods, so you should implement that as well. For example, with empty dummy methods if you are not interested about the progress.

Instead of using QI like above, you can also implement nsIInterfaceRequestor and return nsIAuthPrompt from there, like nsIWebBrowserPersist.progressListener documentation suggests.

Downloading Images

Sample function for fetching an image file from a URL.

// This function is for fetching an image file from a URL. 
// Accepts a URL and returns the file. 
// Returns empty if the file is not found (with an 404 error for instance). 
// Tried with .jpg, .ico, .gif (even .html).
function GetImageFromURL(url) { 
  var ioserv = Components.classes["@mozilla.org/network/io-service;1"] 
               .getService(Components.interfaces.nsIIOService); 
  var channel = ioserv.newChannel(url, 0, null); 
  var stream = channel.open(); 
  if (channel instanceof Components.interfaces.nsIHttpChannel && channel.responseStatus != 200) { 
    return ""; 
  }
  var bstream = Components.classes["@mozilla.org/binaryinputstream;1"] 
                .createInstance(Components.interfaces.nsIBinaryInputStream); 
  bstream.setInputStream(stream); 
  var size = 0; 
  var file_data = ""; 
  while(size = bstream.available()) { 
    file_data += bstream.readBytes(size); 
  } 
  return file_data; 
}

See also

• nsIDownloadProgressListener
• Saving an arbitrary URL to a local file
• Customizing the download progress bar appearance