Mozilla released the next major update for Thunderbird on April 12th, 2016. Thunderbird 45. There are many changes and fixes in this version which can be viewed in the Release Notes. One of the most noticeable changes is the addition of Correspondents column combining Sender and Recipient with an arrow on the left side of the email address.
I don’t understand the purpose of this and even more so after reading Bug 36489 which dates back to almost 16-years ago (April 20, 2000). Frankly, I don’t like the change. It’s not the name change which I could use to, but rather adding the arrow. Good news is there is an easy fix to this, which I will post shortly.
User can NOT currently update from within Thunderbird via Help > About Thunderbird option. This update method is typically disabled during the first few days of a major release. User should be able to update using this method towards the end of this week or beginning of next week. Users still can download and manually install via the getthunderbird.com site. The next scheduled update is Thunderbird 45.1.0 in late April/early May 2016.