Are your other applications and programs working properly in the new monitor? I recently changed to a newer computer and found the most useful, and universally effective adjustment was to set the display dpi in the display settings to match the new high-res screen.

If this trouble is specific to Thunderbird, then we can tweak its own dpi setting.

You might want to take a step back and undo the changes you have used to enlarge the message text, then try this other adjustment that can adjust all the text in Thunderbird.

Go to Tools|Options|Advanced|General→Config Editor

If challenged, assure it you will be careful.

In the search box, type layout.css.dpi

Note its current setting. It will probably be set to -1. Double-click it to change it to 96 and see what happens to Thunderbird's display. Try gradually increasing (or decreasing) the number and look for the effect in Thunderbird. (Mine would need to be set at 142 dpi if I hadn't been able to set the global display scaling elsewhere.) If you can find a setting that makes the size of your menus and message lists acceptable, then OK. If it doesn't help you, return it to its original setting, report back and we'll think again.

Applying this adjustment will also affect the message text size, so you might have to undo any specific message text adjustments that you have already applied.

The traditional response to your question would be to advise the installation of an add-on, but since that very add-on recently stopped working due to a time-out built into it, I'm keen to help users to find another way to do this adjustment.

Changed to 96, made no difference at all. I even closed and re opened it just in case that was needed to make the change, But no difference.

But did you then change 96 to something else?