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Zach Leatherman

Cross Domain XHR with Firefox 2

30 Aug 2007 Zach Leatherman


This article is old and may contain information that is outdated, irrelevant, or—dare I say it—no longer accurate. Read with care!

By now know you know that trying to do an XMLHttpRequest (XHR or AJAX) call to a domain that is different from the domain of the hosted JavaScript in Firefox will throw an exception.

Error: uncaught exception: Permission denied to call method

If you don’t want a history of the past solutions, page down to see the final solution.

The web has solutions to this problem, but most of them involve changing your JavaScript code, which I thought to be less than ideal. A common solution involves setting the UniversalBrowserRead security property in your JavaScript code [Dion Almaer, of Ajaxian fame]:'UniversalBrowserRead');

The problem with that solution (obviously) lies in single browser proprietary JavaScript polluting your code. And you have to set this property inside the scope of any usage (ie: inside your library file that does your AJAX calls and inside your callbacks, etc).

Why can’t it just be as easy as Internet Explorer? They just pop-up a little security dialog asking you if you want to allow this access (which is also what the enablePrivilege function does as well).

Another solution involves setting the preference inside your prefs.js Firefox preference file [Mike Dirolf]. This worked as desired and allowed the AJAX call, but anytime you attempt to access the resulting XML you received a nice exception as well. It turns out this is the solution we wanted, it’s just incomplete.

The Final Solution

  1. Close Firefox. It will overwrite your changes to the prefs.js file if you have it open.

  2. Optional step: This approach will open up your Firefox security quite a bit, so I’d recommend setting up a separate profile in Firefox to use when testing. It will **not **pop up a security dialog when a cross-domain AJAX call is made.

  3. Find your prefs.js file. In Windows, it is typically located in the C:Documents and Settings{YOUR_USERNAME}ApplicationDataMozillaFirefoxProfiles{YOUR_TEST_USER_PROFILE_ID}prefs.js

  4. Open it up and add the following lines:

    user_pref("", "allAccess");
    user_pref("capability.policy.default.CDATASection.nodeValue", "allAccess");
    user_pref("capability.policy.default.Element.attributes", "allAccess");
    user_pref("capability.policy.default.Element.childNodes", "allAccess");
    user_pref("capability.policy.default.Element.firstChild", "allAccess");
    user_pref("capability.policy.default.Element.getElementsByTagName", "allAccess");
    user_pref("capability.policy.default.Element.tagName", "allAccess");
    user_pref("capability.policy.default.HTMLCollection.length", "allAccess");
    user_pref("capability.policy.default.HTMLCollection.item", "allAccess");
    user_pref("capability.policy.default.Text.nodeValue", "allAccess");
    user_pref("capability.policy.default.XMLDocument.documentElement", "allAccess");
    user_pref("capability.policy.default.XMLDocument.getElementsByTagName", "allAccess");
    user_pref("", "allAccess");
    user_pref("", "allAccess");
    user_pref("capability.policy.default.XMLHttpRequest.responseText", "allAccess");
    user_pref("capability.policy.default.XMLHttpRequest.responseXML", "allAccess");
    user_pref("capability.policy.default.XMLHttpRequest.send", "allAccess");
    user_pref("capability.policy.default.XMLHttpRequest.setRequestHeader", "allAccess");

This code was copied (with the exception of 1 line) from a source repository at []
used in jackfield. It wasn’t intended to be used for this purpose, but it works.

If you still get Error: uncaught exception: Permission denied to call method _________ errors, you can add the method to your prefs.js. I would appreciate a comment with any commonly used methods not included above. Thanks.

Update: Because this article is deprecated (applies to an older version of Firefox), I’m updating the blog title in the interest of minimizing the number of disappointed users. Some might think this is a stupid thing to do, since it’s the most popular page on my blog, but I’m more interested in helping people than getting traffic.


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this method works as well but when I need to read the response xml i get this exception:

uncaught exception: Permission denied to get property XMLDocument.firstChild

any solution??
i don't want to use ie6/7 again for this purpose...

Zach Leatherman

30 Oct 2007 at 09:56PM

If you read the solution carefully you will see that you can add that exact property to your prefs.js like so:

user_pref("capability.policy.default.XMLDocument.firstChild", "allAccess");

Anytime you get a "Permission denied" exception, this solution should work

Rather than set these preferences on the default profile, set them on a custom one that's only available for your development site:

user_pref("capability.policy.XMLHttpRequestToAnySite.sites", "http://YOURDEVHOSTHERE");
user_pref("capability.policy.policynames", "XMLHttpRequestToAnySite");

Here's the final list I ended up using, which includes a few new ones than the OP:

user_pref("capability.policy.XMLHttpRequestToAnySite.CDATASection.nodeValue", "allAccess");

user_pref("capability.policy.XMLHttpRequestToAnySite.Element.attributes", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.childNodes", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.firstChild", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.getAttribute", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.getElementsByTagName", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.nodeType", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.nodeName", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Element.tagName", "allAccess");

user_pref("capability.policy.XMLHttpRequestToAnySite.HTMLCollection.length", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.HTMLCollection.item", "allAccess");

user_pref("capability.policy.XMLHttpRequestToAnySite.ProcessingInstruction.nodeType", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.ProcessingInstruction.nodeName", "allAccess");

user_pref("capability.policy.XMLHttpRequestToAnySite.Text.nodeType", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Text.nodeName", "allAccess");
user_pref("", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.Text.nodeValue", "allAccess");

user_pref("capability.policy.XMLHttpRequestToAnySite.XMLDocument.documentElement", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLDocument.getElementsByTagName", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLDocument.nodeType", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLDocument.nodeName", "allAccess");

user_pref("", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLHttpRequest.responseText", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLHttpRequest.responseXML", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLHttpRequest.send", "allAccess");
user_pref("capability.policy.XMLHttpRequestToAnySite.XMLHttpRequest.setRequestHeader", "allAccess");
user_pref("", "allAccess");

Zach Leatherman

06 Feb 2008 at 11:27PM

Great comment Shaun! That will definitely tighten the security up.

Christian Fecteau

10 Feb 2008 at 09:30PM

Shaun or Zach, how would I do the same for UniversalXPConnect? I'm trying to make myself a Firefox favelet that would search this French dictionnary automatically with my clipboard text:

I'm so close but I'm still getting this error:
uncaught exception: Un script de « » s'est vu refuser des privilèges UniversalXPConnect.

I've tried these with no luck in user.js:
user_pref("capability.principal.codebase.p0.granted", "UniversalXPConnect");
user_pref("", "");
user_pref("capability.principal.codebase.p0.subjectName", "");

user_pref("capability.principal.codebase.p1.granted", "UniversalXPConnect");
user_pref("", "http://dell");
user_pref("capability.principal.codebase.p1.subjectName", "");

user_pref("capability.policy.policynames", "allowclipboard");
user_pref("capability.policy.allowclipboard.sites", "file:// http://dell");
user_pref("capability.policy.allowclipboard.Clipboard.cutcopy", "allAccess");
user_pref("capability.policy.allowclipboard.Clipboard.paste", "allAccess");

user_pref("capability.policy.allowclipboard.UniversalXPConnect.QueryInterface", "allAccess");
user_pref("capability.policy.allowclipboard.UniversalXPConnect.createInstance", "allAccess");
user_pref("capability.policy.allowclipboard.UniversalXPConnect.equals", "allAccess");
user_pref("capability.policy.allowclipboard.UniversalXPConnect.getService", "allAccess");
user_pref("capability.policy.allowclipboard.UniversalXPConnect.initialize", "allAccess");

and my favelet:
function getClipboardText() {

if (window.clipboardData) // IE
return (window.clipboardData.getData('Text'));

if (!window.netscape) // promt for text if not Mozilla
return prompt();

// ask permission to Mozilla'UniversalXPConnect');

// go on with XUL code
varclip = Components
trans = Components
str = {},
len = {};

clip.getData(trans, clip.kGlobalClipboard);
trans.getTransferData('text/unicode', str, len);

str =Components.interfaces.nsISupportsWString
? str.value.QueryInterface(Components.interfaces.nsISupportsWString)
: str.value.QueryInterface(Components.interfaces.nsISupportsString);

return(, len.value / 2));

var clip = getClipboardText();


//document.form1.elements.mot.value = clip;


Thanks for the XHR code. I've added it to my FF dev profile. Now I can code cross-domain ajax.

Christian Fecteau

16 Feb 2008 at 02:54AM

Problem solved. A Firefox search engine xml file has been made for this dictionnary (and more):

If you want to ensure that what you are developing works for other users who will not want to do this to their firefox use this:

// IE understands this
var xml = $(response.responseXML)
var codes = xml.documentElement.getElementsByTagName("code")
//firefox understands this
var xml = (new DOMParser()).parseFromString( response.responseXML, "text/xml");
var codes = xml.documentElement.getElementsByTagName("code")

Zach Leatherman

10 May 2008 at 03:25AM

Dude, what?

To me that says:
I can parse an XML document.
OR if I can't, try a new way.

That doesn't have anything to do with XHR, other than supplying using it to supply an XML document.

Zach Leatherman

10 May 2008 at 06:10PM

Also, please keep in mind that this is NOT a production solution. It's a development solution for individual development machines.

Heard this was a viable production solution:
use php to pull the xml, and feed it into the javascript.

Zach Leatherman

18 Jun 2008 at 10:24PM

Sure, this method is a client-side only solution. It's fairly trivial (and common) to use a server side language to act as a proxy for your ajax calls.

hi board!

y have this error,
Permiso para obtener la propiedad Element.childNodes denegado

in english
Permission denied to get property Element.childNodes

and i create prefs.js file, and i have this error yet... i am using V 3.0

tks for u help.

I've read that the above technique will not work under Firefox 3. My tests seem to confirm this. And, there's this thread:

So, is it game over?

Zach Leatherman

12 Aug 2008 at 12:19PM

Never say die.


Looks like you could make a plugin that enables this for the web :) Have at it!


Many thanks to you. But my firefox doent get updated with the new user prefs. im running 3.0.6. Any clues.

Zach Leatherman

12 Feb 2009 at 04:16PM

If you're having trouble modifying your prefs, you probably still have Firefox open. You must close Firefox when editing prefs.js. Also, the above code was tested only with Firefox 2, as stated below the headline.

Sorry, but NONE of this works.

Zach Leatherman

10 May 2009 at 02:58PM

Again, please keep in mind that this post should be considered deprecated, as it does not apply to the current version of Firefox.