FireFox Add-on: retrieve text from textbox in current tab gives no result

Question

I am trying to create an add-on that gets the contents (textbox.value) from a textbox with ID = city from the current webpage and write it to a text file.

Answer 1

Without more information, it is necessary to guess at what your problem is. The most likely issue is that you are attempting to find the textbox element with ID=city in the wrong document.

Firefox add-ons generally run in a scope where the global window object is not defined (if it is defined depends on how the portion of your code that is currently running was entered). Even if it is defined, it is often not defined as the window which you are expecting (the window of the current tab). You will probably need to obtain a reference to the window object for the most recently accessed window/tab.

If a browser window exists (in some instances you could be running where no browser window exists, yet, e.g. at start-up), you can obtain a reference to the most recent browser window, document, and gBrowser with:

if (window === null || typeof window !== "object") {
    //If you do not already have a window reference, you need to obtain one:
    //  Add/remove a "/" to comment/un-comment the code appropriate for your add-on type.
    /* Add-on SDK:
    var window = require('sdk/window/utils').getMostRecentBrowserWindow();
    //*/
    //* Overlay and bootstrap (from almost any context/scope):
    var window=Components.classes["@mozilla.org/appshell/window-mediator;1"]
                         .getService(Components.interfaces.nsIWindowMediator)
                         .getMostRecentWindow("navigator:browser");        
    //*/
}
if (typeof document === "undefined") {
    //If there is no document defined, get it
    var document = window.content.document;
}
if (typeof gBrowser === "undefined") {
    //If there is no gBrowser defined, get it
    var gBrowser = window.gBrowser;
}

If you are running the code in response to an event (e.g. a button command event), you can obtain the current window with:

var window = event.view

The lack of having the global window object available, or having it reference something other than what you are expecting, is something that many people encounter as a problem when writing Firefox add-ons.

Note: If you are wanting to be natively compatible with multi-process Firefox (Electrolysis, or e10s), then gaining access to the contents of the current document is more complex. There are shims in place which should make your code continue to work with multi-process Firefox for some time, but they may/will eventually go away.

References:

1. nsIWindowMediator
2. Working with windows in chrome code
3. SDK: window/utils
4. SDK: windows

Large portions of this were copied from my earlier answers, including this link.