Develop Reference

Iframe access on elements

I use an Iframe with an external API, which I don't control. My goal is to add Javascript code in my Website, to change the style of a few elements in the Iframe. First I considered using
document.getElementById(iframeId).contentWindow.document.getElementById(elementId);
but I soon recognized that it will not be successful because I am getting security issues. I tested a bit and found out when I inspect the element in the Iframe, which I want to change, I am able to get the element simply by using:
document.getElementById(elementId);
I don't understand. Why does this technique only work when I inspect the correct element first? And is there any way I can use this trick for my normal JS backend?

Best way to interact with iframe

Hey guys i really need your help. I am creating some kind of application for dynamically searching multiple items. For example, I want to search specific products in one store via iframe(for example, src will be 'www.store.com/search'), i create a array of all objects which i want to search and i create loop which will change iframe's src('www.store.com/search/item-1', 'www.store.com/search/item-2', etc.). I need to access price for selected item, preferably through iframes dom element. I was trying to access iframes dom element and console with no success (Blocked a frame with origin from accessing a cross-origin frame). I am pretty sure im not headed in the right direction, does anyone have better solution or a way to bypass this?

If you don't control the contents inside of the iframe there is no easy way to do this. The web browser has a security feature (called CORS - https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) to prevent this sort of interaction for security reasons.
In order for this to be possible the page inside of the iframe needs to be your page, and you need to configure the CORS settings on it to allow being used with an iframe like this.

Firefox Addon to manipulate DOM without giving the website access to the changes

I write a Mozilla Firefox Addon, that lets me comment websites: When I open a website and click somewhere, the plugin creates a <div> box at this location, where I can enter a comment text. Later, when I open the website again, the plugin automatically puts my previously created comment boxes at the places they where before. (Similar to a comment feature in many PDF readers, etc.)
This leads to a security problem: A website could use an event listener to listen to the creation of new <div> elements and read their content, allowing it to read my private comments.
How can I solve this security issue? Basically, I want a Firefox addon to put private content in a website, while the website should not be able to access this content via JavaScript. (Unless I want it to.)
I could listen to listeners and detach them as soon as the website attaches them - but that does sound like a solid solution.
Is there a security concept in order to make my addon the authority over DOM changes, respectively, to control the access to certain elements?
Alternatively, would it be possible to implement some sort of overlay, which would not be an actual part of the websites DOM but only accessible by the addon?
Similar security problems should occur with other addons. How do they solve it?

If you inject the DOM in a document, the document will always be able to manipulate it, you can't really do much about it. You can either:
1) Don't inject your comment directly in the document, but just a placeholder were there is the first words of the comment, or an image version of the comment (you can generate that with canvas), leave the full ones in your JavaScript Add-on scope, that is not accessible from the page: when you click to edit or add, you can open a panel instead and do the editing there.
2) Inject an iframe, if you have your page remotely in another domain shouldn't be a problem at all, the parent's document can't access to the iframe; but also viceversa: you need to attach content script to your iframe in order to talk with your add-on code, and then you can use your add-on code to send and receive messages from both iframe and parent's document.
If you use a local resource:// document, I'm afraid you need a terrible workaround instead, and you need to use sandbox policies to avoid that the parent's document can communicate with the iframe itself. See my reply here: Firefox Addon SDK: Loading addon file into iframe
3) Use CSS: you can apply a CSS to a document via contentStyle and contentStyleFile in page-mods. The CSS attached in this way can't be inspected by the document itself, and you could use content to add your text to the page, without actually adding DOM that can be inspected. So, your style for instance could be:
span#comment-12::after{
content: 'Hello World';
}
Where the DOM you add could be:
<div><span id='comment-12'></span></div>
If the page tries to inspect the content of the span, it will get an empty text node; and because from the page itself the stylesheet added in this way cannot be inspected, they cannot the styles rules to get the text.
Not sure if there are alternatives, those are the solutions that pop to my mind.

Add-ons that do similar things implement some combination of a whitelist / blacklist feature where the add-on user either specifies which sites they want the action to happen on, or a range of sites they don't want it to happen on. As an add-on author, you would create this and perhaps provide a sensible default configuration. Adblock Plus does something similar.

Create an iframe and bind all your events to the new DOM. By giving it a different domain to the website, you will prevent them from listening in to events and changes.

Addons can use use the anonymous content API used by the devtools to create its node highlighter overlays.
Although the operations supported on anonymous content are fairly limited, so it may or may not be sufficient for your use-case.

How do I dynamically add a div element to any page from a Firefox extension/Addon

I am writing an extension for firefox which will be used to annotate pages on the web via a service. I would like to have a div or an overlay element (probably XUL based) at the bottom of the page which will let people annotate a page and save it. Something like what the Google Friend Connect does on this page, but via an addon.
This floating div/overlay should show up for every page on FF and should render contents from a web service. How do I start building this out?
If it is possible to access DOM via a FF plugin and alter it, then I would like to be able to add a floating div to the body of the document. But that doesn't work either. Example posted here: Dynamically adding a floating div to a page

There are several things you have to do:
You probably want to add some custom CSS to style the div. You can use the stylesheet service.
You have to attach an event handler to the load event ( or DOMContentLoaded), to be notified when a page finished loading. Have a look at Intercepting Page Loads and On page load.
You need a reference to element you want the new element append to. Tabbed Browser provides some useful information. E.g. you can get a reference to the body of the current selected tab gBrowser.contentDocument.body.
Regarding your code example: You forgot the give the element the CSS property position: absolute; or position: fixed; (you have a typo in your code, you wrote postion), depending on whether it should appear at the bottom of the page or the screen.

You can do this (because I have). To do it you'll need to find the node you want to change the content of (if you're adding to the bottom of the page, you may want to use the <body> node I guess) and then call one of:
insertBefore(theNewNode, afterThisNode);
insertAfter(theNewNode, thisNode);
Or possibly, but I'm not sure:
anExistingNode.innerHTML = anExistingNode.innerHTML + myNewContent;
That should be enough to get you started.

How do I get the current iframe location from within the iframe?

I've seen solutions like
top.document.getElementById('iframe_a').src
But the problem is, that I don't know the Iframes ID. My Application is running as a Gmail-Gadget and therefore I don't know which ID Google will generate.
So how can I get the URL of my gadget?
Using JQuery will also be fine.

If your script is executed within an iframe, location.href should do it. You might want to access the location object in more detail tho.