NatGreen said

Is there any way of contacting someone who may actually know why this has occurred?

Yes. You use this forum. You might want to read the Release Note before or after an upgrade so you can keep informed. https://www.mozilla.org/en-US/thunderbird/45.1.0/releasenotes/

The Arrows are part of the new Correspondents column that combined the From and Recipient columns. If you don't like it turn it off and turn the other two columns back on. At the right end of the column header bar is an icon. Click that icon and set your system up any way you like it.

FYI: The reason that you get to use Thunderbird for free is it is developed by programmers that donate their time. This forum is supported by Thunderbird users that donate their time to answer your questions.

"Correspondents" combines the "To" and "From" columns. Most of us don't need to know that we either sent a message, or it was sent to us, so a column that lists the other parties to an email conversation seems to me to be a perfectly logical arrangement, particularly if you file messages together according to the topic or the context, rather than keeping all your sent messages together in Sent. I don't understand why folk would choose to split a conversation between multiple folders.

Even so, each message in a Sent folder would ordinarily show who you sent it to. So it would have a "Recipients" column. Likewise, your Inbox will contain messages all sent to you (by and large) and will usually have a column for "From". Since in both cases a "Correspondents" column will show just the same thing, it seems to me to be simpler cleaner and more consistent. Where there is any ambiguity you can still add the "From" or "Recipient" column.

Right-click where it says "Correspondents" and a drop-down list will appear, showing all the columns available. You can thereby restore the previous "Recipient" and "From" columns.

The arrows serve to indicate which direction the message travelled; either to you or from you.

I must confess that I am surprised that any of this needs explanation.

Thanks for the replies and explanations on this, and the bit of background on how Mozilla operates. It's far and away the best email programme I've used and I appreciate the work that goes into supporting and improving it. I was thrown by the changes and, as pointed out, hadn't read the background. I've now reverted back to the original, as directed by one of the responses.

Regards, Nat

Mozilla themselves do not have the resources to do a call centre for one on one support by phone, chat or email for their own products and projects like Firefox as it would have been done for free.