Release Notes: There is now support for Atom Threading Extensions, better support for migrating profiles from Thunderbird, and several improvements to the autocomplete and mail/news user interfaces. TLS 1.1 and TLS 1.2 are enabled by default, and support for the SPDY 3.1 has been added. Besides this, there are several other new features, stability fixes, and security fixes.
Release Notes: All plugins are now defaulted to click-to-play. Page load times have been improved. Support for H.264 on Linux, and support for MP3 on Windows XP. The password manager supports script-generated password fields. Download progress is now shown in the Mac OS X app dock icon. EXIF orientation is now being used when displaying attached images in MailNews. MailNews notifications have a new look. Plus several bugfixes and improvements to stability and standards compliance.
Release Notes: This release adds Web Audio and CSS3 background-attachment:local support, many new ES6 functions, and a "Validate this page" command. iframe content can now be specified inline. Sorting messages by date can now be configured to look at the thread root. Plugins doorhangers now allow activating different plugin types independently. The proxy popup is now also available from the MailNews main window. A new Recipients column has been added which shows all recipients. The default HTML5 A/V player can change the playback rate. The Firefox devtools debugger can now be used to debug SeaMonkey remotely.
Release Notes: Implements an option to thread messages received by date. Allows deletion of news posts by default. Implements optional taskbar preview-per-tab. Adds support (permission prompt) for desktop notifications. Adds an Isn't operator for searching by priority. Support for a new scrollbar style on Mac OS X 10.7 and newer. Accessibility-related improvements on using pinned tabs. Major SVG rendering improvements around Image tiling and scaling. Removes support for sherlock files that are loaded from application or profile directory. Support for W3C touch events is disabled.
Release Notes: The Content Security Policy (CSP) 1.0 parser has been enabled. The Mixed Content Blocker has been enabled, blocking insecure active content loads on HTTPS pages. New mail alert information can be customized now (Preferences/Mail & Newsgroups/Notifications). A confirmation prompt has been introduced to protect against accidental permanent data loss when force-deleting messages using Shift+Del. The MailNews Save As Template supports multiple selections now. The size on disk is now shown for newsgroup folders.