SeaMonkey 2.1 contains the following major changes relative to SeaMonkey 2.0.14:

• Sync (f.k.a. Weave) is now included directly in SeaMonkey. Sync allows you to keep your browsing history, passwords, bookmarks, preferences and tabs in sync across different devices (computers) in a secure way.
• SeaMonkey windows can now be themed with Personas (lightweight themes).
• The bookmarks system has been reworked to use the Places framework shared with Firefox (already used for history data in SeaMonkey 2.0), including a new Bookmark Manager and fast bookmarking button in the location bar.
• Find in Page now works with a toolbar instead of a modal window.
• OpenSearch plugins are now supported and the default for web search.
• An optional search bar (including suggestions if the search engine supports it) is available in browser toolbar customization, and an engine manager for OpenSearch is available.
• More toolbar widgets are now customizable.
• The new Add-ons Manager opens directly in the browser now and has received a major facelift.
• The new Data Manager now unifies cookie, permission, password, and form data management.
• Flash cookies (LSOs) will now be removed when using Clear Private Data / Cookies (requires Flash Player 10.3 or better).
• Plugin crashes do not take down the whole application anymore since plugins run in their own processes now.
• Plugins now work in feeds shown in MailNews windows.
• The feed preview UI and the Helper Applications preferences can now detect the system's default feed reader and use it.
• Page zoom is being remembered on a site-specific basis now.
• The browser tab bar is now scrollable to cope with tab overflow. This includes an "All Tabs" navigation button to quickly list all open tabs.
• You can now drag and drop downloads, e.g. from the Download Manager to the desktop.
• SeaMonkey now shows "doorhangers" for notifications like remembering log-in passwords or after installing an extension.
• The new DoNotTrack HTTP header is supported (configurable in Preferences).
• A page with Troubleshooting Information (about:support) and the option to restart in Safe Mode are now available from the Help menu.
• Loading the URL about:memory now shows how much memory is used by different parts of SeaMonkey.
• SeaMonkey should now support more websites that previously appeared to only work with Firefox (configurable in Preferences).
• Built-in extensions (ChatZilla, JavaScript Debugger and DOM Inspector) are now installed into the profile.
• News messages can now be deleted from local storage.
• Cascaded Session Restore improves restoring large browsing sessions.
• SeaMonkey now uses Omnijar which results in fewer installed files, less fragmentation and better startup time.

Platform-specific changes

• Mac OS X 10.4 "Tiger" and PPC support was dropped, 64-bit support for 10.6 "Snow Leopard" was added.
• On Linux, desktop notifications (e.g. mail alerts, finished downloads) show up in the native environment.
• Windows 7 Jump Lists support was added.
• Download progress is now shown in the Windows 7 taskbar.

Mozilla platform changes

• Better performance on startup and shutdown, and memory usage improvements.
• Protection from out-of-date plugins has been added.
• Text areas in web forms are now resizable.
• CSS :visited selectors have been changed to block ways that websites can quickly check a user's browsing history.
• The WOFF format for downloadable website fonts is now supported.
• SMIL animation in SVG is now supported.
• New CSS attributes such as gradients, background sizing, and pointer events have been implemented.
• A new HTML5 parser is used for all HTML documents.
• New DOM and HTML5 features including the Drag & Drop API and the File API are now supported.
• JS-ctypes support has been improved, allowing in-application JavaScript code to access native system libraries.
• HTML5 video can now be viewed full screen and supports both the WebM format and the "buffered" property.
• Web developers can animate content using CSS Transitions.
• The W3C Indexed Database API is now available to websites.
• JavaScript is faster than ever with the new JägerMonkey engine.
• Support for WebGL, Direct2D and Direct3D acceleration on Windows has been added (see about:support for support and Preferences for configuration).
• The HSTS security protocol and the proposed Audio Data API are available.
• OpenType support has been improved.
• HTML5 Forms API support has been added.