You have at least two choices...

1. Instead of clicking on a subfolder, right+click on the folder and use Open in new tab. Then close the tab when you are done with the subfolder. Or, don't close the tab but click on tab #1 which still has the Inbox folder.

2. use "[" key to go back through the message history - https://support.mozilla.org/en-US/kb/keyboard-shortcuts-thunderbird

Wayne, Thank you for the suggestions. Guess I was hoping for a shortcut key. Your suggestions are multiple steps to get back to the Inbox, but good to know.... Thank you, Ruth

Have a look at the quick folders addon.