TB opens by default in the folder that was selected at the last closing of TB. If that isn't working, open the profile folder (Help/Troubleshooting Information, click Open Folder), close TB, and delete or rename session.json. A new file will be created when TB restarts, and the default behaviour should return, so if you close TB with Inbox selected, it should be open when you restart. If you want TB to open in a particular folder, no matter which folder was open at closing, use the Manually Sort Folders add-on. https://support.mozilla.org/en-US/questions/1239651#answer-1170637