Error: uncaught exception: Permission denied to call method FireFox/Mozilla browser fix / solution:

  • Go to address “about:config” in Firefox (i.e. type that in the address bar and hit Enter)
  • Search for “signed” in the filter bar
  • Double click the item “signed.applets.codebase_principal_support” to change its value to “true”
  • Create (or edit if already present) the “user.js” file found in the below directories. By default this file does not exist so create a new blank user.js file if you don’t find it in the following paths (as specified on
    • On Windows Vista/XP/2000, the path is usually %AppData%\Mozilla\Firefox\Profiles\xxxxxxxx.default\, where xxxxxxxx is a random string of 8 characters. Just browse to C:\Documents and Settings\[User Name]\Application Data\Mozilla\Firefox\Profiles\ on Windows XP/2000 or C:\users\[User Name]\AppData\Roaming\Mozilla\Firefox\Profiles\ on Windows Vista, and the rest should be obvious.
    • On Windows 95/98/Me, the path is usually C:\WINDOWS\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default\
    • On Linux, the path is usually ~/.mozilla/firefox/xxxxxxxx.default/
    • On Mac OS X, the path is usually ~/Library/Application Support/Firefox/Profiles/xxxxxxxx.default/
  • Place the following lines within user.js:

    user_pref("", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.sites", "");
    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.lastChild", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Element.nodeName", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Element.nodeType", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Element.parentNode", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Element.tagName", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Element.nextSibling", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Element.previousSibling", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.HTMLCollection.length", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.HTMLCollection.item", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.attributes", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.childNodes", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.firstChild", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.getAttribute", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.getElementsByTagName", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.lastChild", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.nodeName", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.nodeType", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.parentNode", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.tagName", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.nextSibling", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.Text.previousSibling", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.XMLDocument.documentElement", "allAccess");
    user_pref("capability.policy.XMLHttpRequestToAnySite.XMLDocument.getElementsByTagName", "allAccess");
    user_pref("", "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("capability.policy.policynames", "XMLHttpRequestToAnySite");
  • Edit the line containing” and replace that URI with whatever URI you are developing on (or publishing to). For me it happens to be Normally it would be just “localhost” for most people or localhost:3000 for Rails project developers.
  • Save the user.js file
  • Exit out of Firefox or other Mozilla based browser. If on Mac OS X, fully quit Firefox by hitting Cmd+Q, don’t just close the current browser window (which leaves Firefox still running in the background).
  • Launch FireFox again.
  • Exit out of Firefox again. The config file that Firefox actually uses to control the browser is called “prefs.js”, not “user.js”. user.js is the file that we, the end user, are supposed to make changes to, which are then copied over to prefs.js when Firefox is loaded. For whatever reason, the prefs.js file will not be updated with the contents of user.js until you exit Firefox, launch it, exit again (at which point prefs.js will be updated), then launch Firefox once more and your changes are ready for use.

After the above steps are completed, you should be able to make XMLHttpRequest calls cross-site / cross-domain with your AJAX code without Firefox/Mozilla security getting in the way.

The bevy of user_pref settings above creates a new site security policy that allows the listed XML HTTP Request commands to be performed from “” to any address. Normally, Firefox will only allow XMLHTTP Request calls within the same domain. For example if you were on domain, Firefox would not allow the website to make XMLHTTPRequest calls to since this was a major exploit that crackers would use to hide their evildoings in the background of apparently benign sites.

In general the security policy that Firefox has setup by default is a good idea. Setting up a new security policy as we have done above is generally safe as it only allows the site “” to make cross-site/cross-domain XMLHTTPRequest calls of any sort listed. Any other domain would not be allowed to use this site policy.

This post originally started out due to the desire to develop AJAX Toolkit based s-controls outside of their Ajax Tools IDE (yeah, their naming schemes leave something to be desired), which runs on their “no software” platform.  Of course I ran into huge problems with Camino / Firefox and cross domain XMLHTTPRequest scripting security issues.  The result of which is this post on how to get around the cross site scripting issues and develop javascript based s-controls on your local machine, using your preferred IDE (go go Textmate).