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Firefox 23 for developers

Want to help document Firefox 23? See the list of bugs that need to be written about and pitch in!

Changes for Web developers


  • Mixed content blocking. Firefox will no longer load non-secure (http) resources on secure (https) pages (bug 834836).
  • The standard syntax of CSP 1.0 policies are now implemented and enforced by default.

Developer Tools

  • A Network Panel has been added to the developer tools. This is more detailed view than the "Net" view option in the Web Console.
  • Web Console has been renamed "Console", and includes the option to filter security errors/warnings.
  • The new Toolbox Options lets you disable features, change light/dark theme, or enable Chrome/Remote debugging.



  • The Object.defineProperty method can now be used to redefine the length property of an Array object.
  • The option to disable JavaScript, including the options to allow moving windows/replace context menu, have been removed. You may still disable JavaScript by double clicking the "javascript.enabled" option in about:config.




  • Negative widths for the <mspace> element has been implemented (bug 717546).
  • The <semantics> element now determines the visible child as described in the MathML3 specification.

Changes for addon and Mozilla developers

Firefox developer tools

Addons that overlay chrome://browser/content/debugger.xul must now overlay chrome://browser/content/devtools/debugger.xul. You may add references to both these files in chrome.manifest for compatibility.

See also

Older versions

Document Tags and Contributors

 Contributors to this page: Masayuki
 Last updated by: Masayuki,