Thunderbird's normal behaviour is to open as it was when you closed it. So, switch back to the Inbox before closing. I take it that you can do that as you were able to switch to the Sent folder.

I always go to the inbox and close from there, but it still opens at the sent folder. I've looked at all the options on the right hand side.Nothing seems to work. I close using exit and just closing the window. I have shut down the laptop and opened it again. No change

thank you very much. I found the file after a few minutes looking. I renamed it, so I could go back to it if necessary. Closed Thunderbird, then opened it. It started at inbox once again