TB will open in the folder that was open at last closing. If it doesn't, delete session.json from the profile folder, with TB closed.

To set a specific folder that opens, independent of the closing state, there is an add-on:

https://addons.thunderbird.net/en-US/thunderbird/addon/manually-sort-folders/

It does not open to the last open folder, which would be inbox. It always opens with preferences. Close preferences and then it goes to trash folder. Then I have to choose instead inbox, and once opened, then have to click get messages. I used to be able to see inbox immediately with it automatically getting new messages.