New:
When writing a message, a delete button now allows the removal of a recipient. This delete button is displayed when hovering the To/Cc/Bcc selector.
Further improvements to attachments handling during compose: "Attach" button moved to the right to be above the attachment pane. Alt+M now allows to show the attachment pane. The attachment pane can also be shown initially when composing a new message. Right-click on the header to enable this option. It is now also impossible to completely collapse a non-empty attachment pane. That protects against attachments accidentally being sent.
Thunderbird now allows the conversion of folders from mbox to maildir format and vice versa. This is an experimental feature that needs to be enabled by setting the preference mail.store_conversion_enabled.
Light and Dark themes.
WebExtension themes are now enabled in Thunderbird.
FIDO U2F support.
Calendar: Allow copying, cutting or deleting of a selected occurrence or the entire series for recurring events.
Calendar: Provide an option to display locations for events in calendar day and week views.
Calendar: Provide the ability for sending/not sending meeting notifications directly instead of showing a popup.
Changed:
IMPORTANT: Add-ons not marked as compatible with Thunderbird 60 by their authors will be disabled (this can be reverted via preference extensions.strictCompatibility).
Thunderbird will now prompt to compact IMAP folders even if the account is online.
Native notifications on Linux are now re-enabled.
During address entry, the matching part of the address is now shown in bold. Preference mail.autoComplete.commentColumn allows to display the address book where the address is stored.
Calendar: Removal of capability to send email invitations compatible to Outlook 2002 and earlier.
Fixed:
Various problems when forwarding messages inline: Some introduced in Thunderbird 60 beta 7, some dating back to 2005 when using "simple" HTML view.
The IMAP and News subscribe dialog did not present a folder tree (not working since Thunderbird version 59).
Messages moved between IMAP accounts were missing parts (embedded content or attachments) under some circumstances.
Better error handling for Gmail authentication to avoid re-downloading of folders.
Text in the address book card view wasn't selectable.
UTF-7 support (not working since Thunderbird version 56).
Complete fix of the EFAIL vulnerability: 1) Removing some HTML crafted to carry out an attack. 2) Not decrypting subordinate message parts that otherwise might reveal decryted content to the attacker. Preference mailnews.p7m_subparts_external controls this.
Thunderbird used a stale cached password after user edited a saved password.
The new menu item "Add-on Options" introduced in Thunderbird 59 beta is now also working on Mac.
Source view not working for JS, XML, CSS, etc. source files via 'Developer Tools > Error Console'. Other options: 1) Set preference view_source.editor.external and view_source.editor.path to view the source in the editor of your choice. 2) Use 'Developer Tools > Developer Toolbox, Console'. (Not working in earlier versions of Thunderbird 60 beta.).
Calendar: Google account credentials not retained by Lightning.