Firefox 63 Release Overview

The release of the Firefox 63 web browser has taken place, along with the mobile version of Firefox 63 for Android. Additionally, an update for the long-term support branch, version 60.3.0, has been made available. Soon, Firefox 64 will enter beta testing, with its release scheduled for December 11.

Main New Features

• Content Blocking Controls: A new set of settings allows users to manage content blocking. You can enable blocking of all third-party cookies and scripts used for tracking (based on the disconnect.me list). This blocker is currently optional but is planned to be enabled by default in Firefox 65.
• Site-Specific Blocking Status: The address bar now displays a special icon for each site, showing the status of script and cookie blocking. You can add exceptions for sites that do not function properly with tracking scripts blocked.
• WebExtensions in Separate Process on Linux: On Linux, WebExtensions now run in a separate process (already implemented for Windows in Firefox 56 and for macOS in Firefox 61). This isolates extensions, reducing the impact of extension errors on browser stability.
• Experimental AV1 Codec Support: Added experimental support for the AV1 video codec, a royalty-free format that offers better compression than H.264 and VP9. By default, AV1 is disabled (enable it in about:config by setting media.av1.enabled = true).
• US-Specific Start Page Shortcuts: For US users, special shortcuts for Google and Amazon have been added to the start page, along with quick access keys “@google” and “@amazon”. Clicking these shortcuts prompts you to enter a search query directly from the address bar. These shortcuts can be removed or moved like other quick access buttons.
• Exit Confirmation Dialog: When closing the main browser window with multiple tabs open, or exiting via the menu with several windows open, a confirmation dialog now appears.
• Tab Preview with Ctrl+Tab: The tab overview (Ctrl+Tab) now shows window thumbnails instead of icons and scrolls through tabs in the order of most recent use. You can revert to the old behavior in settings with the option “Ctrl+Tab cycles through tabs in recently used order”.
• Accessibility Improvements: Firefox now detects OS accessibility settings and reduces unnecessary animations.
• Removed Features:
  • The “Open in Sidebar” option for bookmarks has been removed.
  • The “Never check for updates” option has been removed from about:preferences; only automatic or user-confirmed update options remain.
• Web Platform Enhancements:
  • Added support for the Shadow DOM specification, allowing multiple DOM trees to be combined and styles to be isolated.
  • Added support for Custom Elements, enabling custom HTML elements that extend existing tags.
  • The img tag now supports the decoding attribute to specify image decoding method (sync, async, auto).
  • Added Object.fromEntries() to convert key/value pairs into an object.
  • Added Symbol.prototype.description property for symbol descriptions.
  • CSS offset-* properties renamed to inset-block-start, inset-block-end, inset-inline-start, and inset-inline-end.
  • Added CSS :defined pseudo-class for any defined element.
  • Added Media Capabilities API for information on encoding/decoding capabilities and performance statistics.
  • Added Async Clipboard API for asynchronous clipboard access using Promises (e.g., const data = await navigator.clipboard.readText()).
  • Added SecurityPolicyViolationEvent interface for Content-Security-Policy violation events.
  • Enabled more Web Animations API properties and interfaces by default: ready, finished, effect, KeyframeEffect, and AnimationEffect.
  • Implemented Element.toggleAttribute() to toggle boolean attributes.
  • Added Window.event property; removed Window.back() and Window.forward() methods.
  • The microphone can now be accessed simultaneously in different tabs.
• Developer Tools Updates:
  • Updated menu styling in developer tools.
  • Added a built-in font editor in the inspector to view and manage fonts, including variable fonts.
  • In Network Monitor, a crossed-out shield icon highlights requests to known tracking URLs.
  • Accessibility inspector is now enabled by default, showing which Accessibility APIs are used and what information is available.
  • System add-ons are no longer shown on the about:debugging page (to show them, set devtools.aboutdebugging.showSystemAddons in about:config).
• Platform-Specific Improvements:
  • Windows: Builds now use the Clang compiler, improving performance. The browser theme now adapts to Windows 10’s light and dark modes.
  • macOS: Improved interface responsiveness and faster tab switching. WebGL now allows GPU selection via the powerPreference attribute, enabling use of lower-power GPUs for less demanding applications.
  • Android: Added Picture-in-Picture video playback, notification channel support, and additional performance and security optimizations for Android 8.0 “Oreo”.
• Security Fixes: Firefox 63 addresses a number of vulnerabilities, several of which are marked as critical and could allow code execution when opening specially crafted pages. Detailed information on the fixed security issues will be published soon.