Thunderbird 65.0 Beta 1

Thunderbird

Thunderbird is a free, open-source, cross-platform application for managing email and news feeds. It is a local (rather than a web-based) email application that is powerful yet easy-to-use. Thunderbird is developed, tested, translated and supported by the folks at Mozilla Corporation and by a group of dedicated volunteers. Thunderbird gives you control and ownership over your email. There are lots of add-ons available for Thunderbird that enable you to extend and customize your email experience.

Thunderbird gives you IMAP/POP support, a built-in RSS reader, support for HTML mail, powerful quick search, saved search folders, advanced message filtering, message grouping, labels, return receipts, smart address book LDAP address completion, import tools, and the ability to manage multiple e-mail and newsgroup accounts.

Thunderbird 65.0 Beta 1 changelog:

What's new

FileLink for WeTransfer
WebExtensions FileLink API to facilitate FileLink add-ons. WeTransfer is already included and Dropbox is available as add-on.
Various theme improvements including a dark message list/thread pane
When multiple language packs are installed, the UI language can now be selected in the Advanced Options. Preference intl.multilingual.enabled needs to be set (and possily also extensions.langpacks.signatures.required needs to be set to false)

Changed

ADD-ON SUPPORT: Restartless add-ons continue to work if their authors have made the necessary minor adjustments. Non-restartless legacy add-ons using XUL overlays are only supported if add-on authors have adapted them. Apart from Lightning/Calendar there are a few that have been adapted, like "ThunderHTMLedit", "Compact Header" and "Signature Switch". Instaling legacy add-ons now requires a restart. Legacy add-ons can now specify an options page again.
Reply to self will now search all identities (default of preference mailnews.reply_to_self_check_all_ident changed to true)
Composition text and background colors no longer sent by default. New option in "Tools > Options, Composition".

What's fixed

Address book search and auto-complete slowness introduced in Thunderbird 64 beta
Plain text markup with * for bold, / for italics, _ for underline and | for code did not work when the enclosed text contained non-ASCII characters
While composing a message, a link not removed when link location was removed in the link properties panel
Decoding problems for messages with less common charsets (cp932, cp936)
Spurious (encoded newline) inserted into drafts and sent email
Double-clicking on a word in the Write window sometimes launched the Advanced Property Editor or Link Properties dialog
Editing a plaint text draft displayed the entire message as quote (in blue)
New messages in the drafts folder (and other special or virtual folders) will no longer be included in the new messages notification
Edit > Undo didn't undo moving or deleting of messages
Improvements of SMTP 5.7.1 message

Known Issues

Account creation sometimes shows an error: "Programming bug. Assertion failed, see log". Workaround: Click "Advanced config" button.
Due to changes in the Mozilla platform profiles stored on Windows network shares addressed via drive letters are now addressed via UNC
Spellcheck marks disappeared erroneously for words with an apostrophe (working in Thunderbird ESR)
Calendar: A localized version of Provider for Google Calendar not available at addons.thunderbird.net or the Mozilla FTP server. A US-English version can be downloaded here.
Chat: Twitter not working due to API changes at Twitter.com
Chat: Participants not sorted