Just how many tabs with open message have you got open. Perhaps you need to right click in the tab and select close all other tabs from the menu. While there is no technical limit on the number of tabs you can have open, they each consume system resources and ultimately may affect how fast your computer can run.

However Alt+1 is the shortcut the spaces toolbar suggests when I hover over the icon that takes you back to the inbox and folders.

This "upgraded" thunderbird (really downgraded?) opens a tab every time I open an email. How do I get it to stop that? And just open one message in a tab if I want one? Isn't that how it worked before?

Sometimes when I click and stay on the arrow at the left it moves past the tabs to the inbox, and sometimes it doesn't, apparently.

Thanks for your reply. I know contributors are trying to help. But why can't I reach the people who are programming and designing thunderbird directly to get them to change something or explain something?