Firefox 33 for developers

Firefox 33 was released on October 14, 2014. This article lists key changes that are useful not only for web developers, but also Firefox and Gecko developers as well as add-on developers.

Changes for Web developers

Developer Tools


  • Event listeners popup
  • @media sidebar
  • Add new rule
  • Edit keyframes
  • Cubic bezier editor
  • Transform highlighter
  • Persistent disable cache
  • New Commands
  • Editor preferences
  • WebIDE

For details please see the hacks post. Special thanks to the 33 contributors that added all the features and fixes in this release.



  • Added the experimental support for <picture> element (Firefox bug 870022), behind the dom.image.picture.enabled preference (off by default).
  • The <label>, especially without a for attribute, doesn't apply anymore to a <input type=hidden> field (Firefox bug 597650). The previous behavior wasn't spec-compliant.
  • The link annotation noreferrer has been implemented on <a> elements. <a rel="noreferrer"> will not include the URL of the referrer in the HTTP request sent to fetch it (Firefox bug 530396). Note that this works only for in-page links, not for links clicked via the UI, like via contextual menus.
  • On Android, support for two new values for the name attribute of <meta> has been added: msapplication-TileImage and msapplication-TileColor (Firefox bug 1014712). Example:
    <meta name="msapplication-TileImage" content="images/benthepcguy-144.png" />
    <meta name="msapplication-TileColor" content="#d83434" />





No change.



  • EXT_blend_minmax is now exposed. It extends blending capabilities by adding two new blend equations: producing the minimum or maximum color components of the source and destination colors (Firefox bug 973815).


Changes for add-on and Mozilla developers

  • The JavaScript Debugger Service (JSD) has been removed in favor of the new Debugger API (Firefox bug 800200).
  • The interface nsIX509CertDB2 has been removed and the methods from that interface have been moved to the nsIX509CertDB interface.

Add-on SDK


  • Added support for context menus in panels via a new option in the Panel constructor.
  • Added tab.readyState.
  • Added a BrowserWindow parameter to and sidebar.hide(), to control the window for which the sidebar will be shown or hidden.


GitHub commits made between Firefox 32 and Firefox 33. This will not include any uplifts made after this release entered Aurora.

Bugs fixed between Firefox 32 and Firefox 33. This will not include any uplifts made after this release entered Aurora.

Older versions