What exactly is a "legacy" GUI? What's your baseline for "no change"?

I use add-ons to restore a look-and-feel that might seem old-fashioned to some. I like the traditional menu across the top.

Legacy graphical user interface is essentially the same as "Classic View" that many applications use, just different verbiage. We'd be looking for the same menu buttons in the left folder pane and menu bar (inbox, sent, delete, compose, etc.), same method for sending pictures, no unnecessary automated "helpful" functions like word-replace (unless they can be easily disabled). Mozilla lists numerous new features that make it sound like the GUI is a lot different, which may or may not be the case. It would be helpful if they show the new look with the description. Upgrade notes] www.mozilla.org/en-US/thunderbird/52.0/releasenotes/

It's hard to judge which, if any, of your list there would be significantly different. I don't recognize any of the things you describe as having substantially changed in the lifetime of Thunderbird.

Items that do tend to irritate users are the addition of Chat and the introduction of tabs, but both have been there since TB17 or thereabouts. That's why I ask what your baseline is.

A major change was the addition of the Application Menu, but this is entirely optional, can be removed and the traditional menu can be reinstated. I suggest that only you can judge which, if any, of these changes are acceptable.

My own version is significantly customized with colourful icons and a traditional menu, and controls moved from the message pane to the main toolbar.

Again, only you can decide which adjustments you would need to make, and whether the sum of changes would be more or less traumatic than a wholesale shift to a different client.

I don't think pictures of the new layout are of any great value to you; what will matter is the possibility of restoring or reverting to a more familiar look'n'feel. In this respect, Thunderbird is the most customizable email client I have ever worked with.

BTW, the row of tabs across near the top (Recent Folders, Unknown etc) are not a standard feature. These are provided by the QuickFolders add-on so don't appear in a standard build.

Likewise, I use the Enigmail and gContact sync add-ons which have added their own items to the menu bar.

Whilst Chat is active in my installation, you can see that it doesn't make itself visible in the menu or toolbar. Again, where it did appear it was possible to remove via customization.

And I have moved features that I wish to have continuously visible such as the Address Book, Calendar and the Application Menu button into the title bar at the top. The only non-customizable items in the title bar that can't be easily dispensed with are the calendar and its associated Events and Tasks buttons, but these will not be present if the calendar is disabled or uninstalled. So you can have as near as possible to a standard title bar as your OS will allow; in this case, Windows 10 limits my range of choices.

The Correspondents column is new, but you can restore the previous From and Recipient columns.

Thank you. Your summary was a big help. I'll upgrade her program, see how it looks, and de-tune it if necessary. Your point about the trauma of switching to another program is well taken. That change would likely be more difficult. She's always liked Thunderbird and is comfortable with it, so I think that's the way to go.

If it is not acceptable after your efforts, you can get old versions here:

https://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/

As always, there is the cautionary advice that old security flaws may be present in older versions.