Mozilla announced this week the availability of Thunderbird 11, a new version of the popular open source e-mail client. It brings a minor user interface overhaul and a number of bugfixes.

As we reported when Thunderbird 5 launched last year, the mail client has transitioned to the same rapid release model as Firefox. Thunderbird is built on top of Gecko, the same rendering engine that Firefox uses to draw its user interface and render HTML content.

Aligning with the Firefox development schedule allows the builders of Thunderbird to continue using the same shared components without having to maintain a fork. Due to the transition to a shorter release cycle and more incremental development model, users can expect to see fewer changes in each individual Thunderbird update. Mozilla says that it will discontinue support for the legacy Thunderbird 3.1.x series in April.

In Thunderbird 11, the application's user interface was overhauled to move the tabs to the top of the window, a design that Mozilla has already previously adopted for Firefox. Unlike Firefox, however, Thunderbird doesn't provide an option that allows users to move the tabs back below the navigation bar.

Mozilla's Mike Conley explained the rationale for moving the tabs in a comment posted to the Mozilla bug tracker. Moving the tabs above the toolbar will allow tabs with different kinds of content to display their own contextually relevant toolbars. This will be advantageous for projects like Lightning, which are adding calendaring capabilities to Thunderbird.

It's also going to simplify the user interface as Mozilla works to integrate other communication tools into the e-mail client. Mozilla has already started prototyping a new built-in instant messaging system for Thunderbird, which is available for testing in the latest nightly builds.

According to some documentation in the Mozilla wiki, the developers plan to expose contact presence information in the e-mail interface and make it easier for users to track conversations that are spread across e-mail and instant messaging. The instant messaging feature could appear in Thunderbird 13.

Thunderbird 11 is available for download from the Mozilla website. For more details, you can refer to the official release announcement.