Note: Not all mail providers allow this functionality.
When using more than one spell check dictionary, the dictionary can now be selected while entering the message subject. There is also a language indicator showing the selected dictionary.
The font size selection has changed, there is now a button for size selection on the editing toolbar and the "Smaller/Larger font size" buttons now change the size to the next available font discrete size.
The Thunderbird composition window now functions more like a word processor in that pressing the "Enter" key inserts a new paragraph, pressing "Shift+Enter" inserts a new line. This new behavior can be switched off in the Thunderbird > PreferencesTools > OptionsEdit > Preferences > Composition > General tab.
Fonts used in a received message that are not locally installed will now be shown in the font indicator.
Mail Delivery Format
Previously, Thunderbird sent mail as plain text if no "rich text" elements were used in the message and the recipient wasn't marked as preferring HTML email in the address book. This automatic "downgrade" can now be disabled in the Thunderbird > PreferencesTools > OptionsEdit > Preferences > Composition > General tab > Send Options… dialog.
Chinese, Japanese and Korean mail
When sending email which was composed using Chinese, Japanese or Korean characters, unwanted extra spaces were inserted within the text. This has been corrected and mail will be sent and received exactly as it was composed.
Japanese mail now fully complies with RFC 1468 and RFC 3676.
Remote Content Exceptions
By default, Thunderbird does not display images embedded in HTML messages that are not sent with the message but need to be loaded from a website. This protects the users against tracking. The menu to select sites for which remote content will be loaded has been improved. (Note: Due to internal changes, remote content exceptions based on the sender's email address are not migrated from Thunderbird 38.)
The headings in the message pane can now show a "Correspondents" column instead of the "From" and "Recipient" columns.
In version 45.0, existing folders will be upgraded to the new column, unless this upgrade is switched off in the "config editor" by setting the mailnews.ui.upgrade.correspondents preference to false. Starting in version 45.1, there won't be any automated upgrade, so users who wish to use the "Correspondents" column by default need to set the mail.threadpane.use_correspondents preference to true.
To access the config editor, go to Thunderbird > PreferencesTools > OptionsEdit > Preferences, select the Advanced panel, select the General tab and click Config Editor…. On the warning page that opens, click I'll be careful, I promise! to proceed to the config editor. For detailed information, see Config Editor.
When viewing a message, the name and email address of sender or recipient can now be copied from the message header using a new option in the context menu.