Firefox 13 for developers

• Tables' cellspacing attributes are now parsed the same outside quirks mode as they are in quirks mode. That is, if a value is specified as a percentage, it's treated as a number of pixels instead, since percentage values are not actually permitted according to the specification.
• The <wbr> element has seen its bidirectional behavior fixed. It now behaves like the Unicode U+200B ZERO-WIDTH SPACE and therefore doesn't affect bi-directionality of its parent element anymore.
• The :invalid pseudo-class can now be applied to the <form> element.

• The turn <angle> unit is now supported (to be used with CSS functions like rotate()).
• Support for 3-to-4 value syntax of the background-position has been added. You can offset a background image from any corner by writing like "right 10px bottom 20px". See Firefox bug 522607
• Support for the 2-value syntax of the CSS background-repeat has been added.
• Support for -moz-border-radius* and -moz-box-shadow has been removed. Authors should use unprefixed border-radius or box-shadow instead. See Firefox bug 693510
• The column-fill property has been implemented (prefixed).

• Support for the ECMAScript 2015 for...of construct has been added.
• Experimental support for ECMAScript 2015 Map and Set objects has been implemented.

• The Node.cloneNode() method's deep argument is now optional, as specified in DOM4.
• The setTimeout() and setInterval() methods no longer pass an additional "lateness" argument to the callback routine.
• The Blob.mozSlice() method has been unprefixed.
• Support for the Blob constructor has been added.
• Support for globalStorage has been removed.
• The new DOMRequest interface, used for reporting the status and result of background operations, has been added.
• The HTMLOptionElement.index() method now returns 0 instead of the incorrect -1 when the <option> is inside a <datalist> HTML element.
• DOMException as defined in DOM Level 4 has been implemented.
• The FileError interface has been removed in favor of the DOMError interface as defined in the latest FileAPI specification.
• The Range object no longer throws a RangeException. Instead a DOMException as defined in DOM 4 is used.
• element.getAttributeNS() now always returns null instead of the empty string for non-existent attributes. Previously, there were cases in which the empty string could be returned. This is in keeping with the DOM4 specification, which now says this should return null for non-existent attributes, instead of an empty string.
• The HTMLCanvasElement interface now has a non-standard mozFetchAsStream() method, which provides an input stream containing the element's image data in the specified format.

• Firefox for Android now has a Tablet or Mobile token in the UA string to indicate the form factor and no longer has the Fennec token. Also, the number after "Gecko/" is now the Gecko version number instead of a frozen date.
• The UA string no longer exposes the Gecko patch number or release status in the version number; that is, the version number is now always of the form "X.Y", where X is the major release number and Y the minor. For example, "13.0" or "14.1". It will no longer be something like "14.0.1b1".

• The SVGStringList DOM interface is now indexable like Array (see Firefox bug 722071).

• Support has been added for the EXT_texture_filter_anisotropic extension. Anisotropic texture filtering improves the quality of mipmapped texture access when viewing a textured primitive at an oblique angle.

• Support for the width attribute on <mtable> elements has been added (Firefox bug 722880).
• MathJax fonts are now used as the default fonts for mathematical text. See Fonts for Mozilla's MathML engine for more information.

• The SPDY protocol now enabled by default.

3D view improvements

• You can now press the "f" key to make sure the currently selected node is visible.

Style panel improvements

• Clicking the heading for any rule in the style panel now opens the Style Editor at the corresponding CSS.
• Right-clicking on a rule in the style panel now offers an option to copy the rule to the clipboard.
• Entering an unknown property name, or an illegal property value, displays a warning icon next to that property.

Scratchpad improvements

• The Scratchpad now has an option in the Help menu to take you to the MDN documentation about Scratchpad.

Starting in Firefox 13, Firefox for Windows requires at least Windows XP Service Pack 2; it will no longer run on Windows 2000 or earlier versions of Windows XP.

source-editor.jsm

• Support for a dirty flag has been added to the Source Editor API.
• The Source Editor no longer supports falling back to a <textarea> instead of using Orion.
• The editor now exposes focus and blur events.
• The getIndentationString() method has been added; this returns the string to use for indenting text in the editor.
• The Source Editor now supports managing a list of breakpoints and displaying user interface for toggling them on and off; it does not actually implement breakpoints, however. That's up to you to write debugger code for.
• Support has been added for highlighting the current line, using the highlightCurrentLine configuration option.

• The CSS properties margin-left, margin-right, margin-top, margin-bottom are now all reflected into ARIA object attributes with the same name. See Gecko object attributes for more information.

• The nsIScreen interface now supports controlling rotation through the new rotation attribute.
• The nsIPrefBranch2 interface has been merged into nsIPrefBranch (Firefox bug 718255).
• The new message manager wake-up service, implemented by nsIMessageWakeupService, has been implemented. See Firefox bug 591052.
• The aliases MozOpacity, MozOutline, MozOutlineStyle, MozOutlineWidth, MozOutlineOffset, and MozOutlineColor, all of which were removed in previous versions of Gecko, have been removed from nsIDOMCSS2Properties, which should have been done with the aliases were initially removed.
• The nsINavHistoryQueryOptions attribute excludeItemIfParentHasAnnotation has been removed, along with the corresponding query operation. It existed to support livemarks, which no longer exist.