See this topic: https://support.mozilla.org/en-US/questions/1301546

Sorry to say but the suggestion does not work. Instead I found this: The only way to get close to the old behavior is to copy the Location of a folder into the Start page Location. So it always opens the folder without the Start Page. Where do I find the location of the Inbox-folder? /hans peter

The method described in the link in my first reply works, but if it doesn't, reset session.json by deleting the file from the profile, while TB is closed. Help/Troubleshooting, click Open Folder to open the profile.

The Inbox folder is in the account subfolder of the Mail or ImapMail folder in the profile.

Sorry, but I need further help. What does reset session.json by deleting the file from the profile mean?

The Inbox folder is in the account subfolder of the Mail or ImapMail folder in the profile. I have found this. (Ooops, I tried to enclose a picture but did not succeed.)

Folder Mail 2017-11-28 15:41 Folder mail.tele2-3.se 2010-08-31 11:11 File Inbox 2020-08-31 11:11 912 780 kB

K.r. /hans peter

The session.json file stores the location of the folder that was open at last exit, as well as other items related to the pane layout and opened tabs. Sometimes it becomes corrupted, but if you delete it while TB is closed, it will be automatically recreated when TB restarts, and may resolve the startup folder issue (without having to specify the folder in the Location box).

Thank you,sfhowes. Excuse my ignorance, but where can I find the session.json file? K.r. /hans peter

It's in the profile folder. See my second reply for instructions.

Yes, I have read it and also made a comment on it, but I have no such file as I can see. /hans peter

Now I have finally found session.json immediately after clicking on Open folder. I was further ahead looking. I did as you said and removed the existing one. After a restart I got a new one and now I am happy again. Thanks so much. Kind regards. /hans peter