Mozilla Thunderbird 91 is an open-source, free cross-platform email, news, calendar and chat client. The new version 91 is a very important update as it brings numerous new features and improvements.
Thunderbird 91 comes a little over a year after the series Thunderbird 78 and brings a refreshed account setup guide with support for creating CalDAV signatures, encryption and calendars, a new user interface for adding attachments, the ability to change the order of accounts in the UI, and the ability to redirect emails.
This new version of Thunderbird also allows you to encrypt your emails to BCC recipients. New keyboard shortcuts are available to access the to, CC and BCC fields of the composition window and allow blank CC and BCC lines to appear in the composition window.
PDF.js is now included by default, and Thunderbird now runs by default in multi-process mode (e10s). Additionally, there is initial support for Matrix servers in the Chat component, and the Tasks component supports undo and restore to create or delete events and tasks.
There is a new context menu in the authoring window that allows you to expand the mailing list box in the recipient list, and two pop-up alerts have been added to notify users when a public recipient of an email exceeds the limit if you select reply to a non-existent email address.
Calendar has many improvements, such as calendar view and universal notification settings, support for opening .ics calendar files with double-clicking, automatic detection of remote calendars, filtering and sorting of imported items in the Insert dialog and a new action in the context menu of the event.
Additionally, Calendar is now the default in CalDAV calendars if supported by the server; it automatically opens the summary dialog box when an existing event is opened; it automatically informs the host operating system that it knows how to handle webcal.
Thunderbird 91 brings native support for Apple M1 devices, support for non-ASCII characters in recipient addresses, Quick Find support for multi-message display (thread summary), and support for pinning folder views in the folder window.
Thunderbird 91 improved support for GMail accounts by adding permissions to calendars and address books without the need for re-authorization, and improved the folder color scheme for better readability.
Last but not least, Thunderbird 91 updated its corporate policies and print dialog box, preselected the IRC server for new chat accounts in Libera Chat, removed Movemail support and the WeTransfer FileLink provider, and moved its UI customization controls in the View menu.
Thunderbird 91 is available for download from here but it is not for upgrading from the Thunderbird 78 series, a feature that will be applied in future updates.