How to scale down display in Thunderbird 153.5.1esr. Huge size on on Linux Debian after TB update.

Sure it's not your distro?

Also, does it reproduce with Help > Troubleshoot Mode?

Wayne : many thanks for your input. I appreciate your assistance. I am not sure that the fault lies with TB. However, I have used Debian-KDE Plasma - Firefox for a long time and TB behaved normally. It is only in the last week that I found the problem. (I am not sure exactly when it occurred as I do not open TB every day.) Starting TB in Troubleshooting mode did not change the huge display. I attach a screenshot to illustrate the problem. Note also that when I try to move the boundary between the left and middle pains to create more space for the message content, the mouse pointer changes to the symbol to show that the boundary has been grabbed, but it will not move. If you can provide any further help I will be grateful. Tom.

There is a minimum panel width for both message list and folder pane. Are you using View > Layout > Vertical?

Wayne. I was using Classic View. When I changed to Vertical View, nothing much seemed different. The huge display persisted. I was really expecting that there would be many controls in Layout that would facilitate scaling, but it seems not to be the case. It is a mystery! Do you think that an uninstall-reinstall might reset the scale? It is a fag to do, re-setting accounts and passwords, but if that is a solution, so be it. Thanks again, Tom.