TBird opens to options page instead of inbox

This last few days, Thunderbird opens in two tabs, with the prevailing tab being the options page. For ten years, at least, I’ve had it open to the inbox. Can someone suggest what setting I altered to change the behavior?
Thank you.

It should open the way it was when last closed. If not, open the profile folder from Help/Troubleshooting, Profile Folder, close TB, delete session.json. Restart TB a few times and see if it opens with the previous layout and open tabs.

Perfect. Thank you. No idea how it got messed up, but am sincerely grateful that you were kind enough to show me how to fix it.