General

• Remove support for all NPAPI plugins (except Flash). Read this blog post for more information.
• DnD of multiple shortcuts from desktop only opens one. This enables drag and drop of multiple links to the browser area, which includes some changes like replacing browserDragAndDrop.drop with browserDragAndDrop.dropLinks.
• The ChromeFilePropertyBag versions of the File constructors are web-observable. This replaces the File constructors with the static functions CreateFromFileName and CreateFromNsIFile.
• Drag and Drop: DataTransfer.types is wrong type. Instead of
  dataTransfer.types.contains, add-ons should use dataTransfer.types.includes. A shim was added to preserve compatibility, but it’s temporary.
• Crash with Youtube Subtiltle Downloader. This change makes CORS requests from add-ons using an expanded principal not work.
• Deprecate and remove CanvasRenderingContext2D.mozDash/mozDashOffset.
• Remove web content access to Battery API.

XPCOM and Modules

• Delete toolkit/obsolete. There are some utility objects there used by older add-ons, like
  nsPreferences in nsUserSettings.js and the string bundle functions in strres.js.
• Mark nsISupportsArray, nsICollection, nsIEnumerator as deprecated. These interfaces, particularly nsISupportsArray, are used in many components add-ons use. They are being replaced with nsIArray:
  • Remove nsISupportsArray usage from nsIDragService.
  • Convert inIDOMUtils.getCSSStyleRules to nsIArray.
  • Use nsIArray for URL window argument.
  • Remove nsISupportsArray usage from nsITransferable and nsIFormatConverter.
• ImageDocument::ImageResizingEnabled is unused.

Themes

• Remove support for obsolete -moz-images-in-menus and -moz-images-in-buttons media queries.