Is the Folder Pane enabled under View/Layout?

The reply was useful but the general layout is poor in comparison with how it was a few weeks ago. Shows only a few messages in the inbox and the folders pane is extremely narrow. Can I revert to the earlier version?

The folder pane can be made wider by clicking and dragging the separator between the pane and the message list. Check that you don't have any filters applied in the Quick Filter Bar and View/Threads is All.

If you try to revert to an earlier version, your current profile will be incompatible, but if you create a new profile, you'll have to add the accounts. Not recommended unless you're familiar with profile management.

Thank you for your advice - very helpful. But I would like to revert to the earlier version. How would I do this, and create a new profiler?

The installer for the previous version can be downloaded here:

https://archive.mozilla.org/pub/thunderbird/releases/115.14.0/win64/

If, on first run, it doesn't start with a new profile, one can be created from Help/Troubleshooting Info, about:profiles. In fact, if you decide to stay with TB 128 but wish to create a new profile, the method is the same.

Thank you for this advice, but when I tried that web address all I got was an index of files, with no mention of a download. Alternatively, is there a method of adjusting TB 128 to return the layout of the inbox page (and other folders) to a column on the left listing all messages in the inbox with approximately half of the page on the right for loading a selected message?

The supplied link shows links for each language, e.g. for the US English version, click on en-US.

Choose a layout from View/Layout, including the Message Pane.