As a fellow user I have learned how to configure some of these issue. First, however, version 115 is an entirely new version which has many new features, but some obvious differences from the prio version. Taking your questions in order:

"Icon on desktop changed" As I understand it, that is intentional to clearly indicate this wholly new version.

"The left pane no longer shows the number of messages in a folder--only the number of unread messages"

Far right of the TBird toolbar (Get Messages, Write, Reply, ...) is a three-bar "Hamburger" menu icon. Click it and then in the popup menu select View, then Folders, and then Folder Pane Header. Now you will have a Header line at the top of the list of folders with the three dots.

Click that three dots icon and select "Show total message count". That will show the total number of messages in each folder (read and unread).

"message filter, the window is too big"

Sorry, I do not know how to adjust that. Mine can get bigger or smaller, but it will only get so small. At its smallest for me it shows 14 filters.

Maybe someone else will have an answer for that.

Thanks, Michael. Regarding: DT: >>message filter, the window is too big<< Michael: >Sorry, I do not know how to adjust that. Mine can get bigger or smaller, but it will only get so small. At its smallest for me it shows 14 filters.<

The problem is not the list of filters--although unlike you, I can't adjust the size of that window either. It is when I edit a single filter (or create a new one). There is no way to get to the bottom of the window to save any changes. That window is too big and cannot be adjusted. Do you know a way of adjusting the size of that window?

Is there a way to force it back to a previous version?

I also wonder if the 32-bit version is a problem, when this is a 64-bit machine.

-DT

64 or 32 bit - don't matter, not a factor.

On windows 11, is your screen set to the default resolution and DPI? Those sometimes cause buttons to push outside a dialog window.

For general information see https://support.mozilla.org/en-US/kb/unified-toolbar and https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova

Until you get a solution, if you finish defining your filter, then click your mouse somewhere new. Now the ENTER key will save the new or edited filter, and the ESC key will cancel the window.

At least this way you can proceed.

Run in safe/troubleshoot mode (hold Shift when launching TB) to test for a broken layout. If the window looks correct in safe mode, launch again in safe mode, 'Reset toolbars & controls', 'make changes & restart'.

After editing the filter itself, hitting enter (THANK YOU Michael FOR THIS, a life saver) saved and then closed the window in one swell foop . . still pretty frustrating that the "size" command brings up the same cursor as "move" and won't do nuthin :)