Firefox 23 for developers

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Changes for Web developers

Security

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.

HTML

The <blink> element support is now completely dropped. The <blink> tag now implements the HTMLUnknownElement interface (bug 857820).
The range state of the <input> element (<input type="range">) has been switched on by default (bug 841950).

JavaScript

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.

DOM

D3E KeyboardEvent.key is now supported, but only for non-printable keys (bug 842927).
The title attribute of DOMImplementation.createHTMLDocument is now optional as per updated DOM specification.
The ability to add a sidebar panel (window.sidebar.addPanel) has been dropped (bug 691647).
The unprefixed Window.requestAnimationFrame and Window.cancelAnimationFrame methods has been added (bug 704063).
The callback for Window.requestAnimationFrame now receives a DOMHighResTimeStamp as argument instead of the less precise DOMTimeStamp used in the unprefixed version (bug 753453).
The text argument for window.alert and window.confirm is now optional (bug 861605).
The HTMLMediaElement.initialTime property, removed from the spec, is no more supported (bug 742537).
The AnimationEvent() constructor has been added (bug 848293).
The AnimationEvent.pseudoElement property has been implemented (bug 848293).
The TransitionEvent() constructor has been added (bug 848291).
The TransitionEvent.pseudoElement property has been implemented (bug 848291).
The non-standard TransitionEvent.initTransitionEvent() and AnimationEvent.initAnimationEvent() have been removed (bug 868751).

CSS

The blink effect for text-decoration: blink; has no more effect, but is still a valid value (bug 857820).
In-flow ::after and ::before pseudo-elements are now flex items (bug 867454).

MathML

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.