Firefox 23 for developers

Changes for Web developers


  • Mixed content blocking. Firefox will no longer load non-secure (http) resources on secure (https) pages (Firefox 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.



  • Instead of including usernames in the RTCIceServer.url property (such as, you now need to use the new RTCIceServer.username property.


  • The blink effect for text-decoration: blink; has no more effect, but is still a valid value (Firefox bug 857820).
  • In-flow ::after and ::before pseudo-elements are now flex items (Firefox bug 867454).
  • The way to compute viewport units has been changed. In conjunction with overflow:auto, space taken by eventual scrollbars is not subtracted from the viewport, whereas in the case of overflow:scroll, it is. (Firefox bug 811403)


  • Negative widths for the <mspace> element has been implemented (Firefox 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