Check you have this setting: 'Menu icon' > 'Folders' > 'All'

mcraigk said

I have a number of email accounts in my thunderbird client. All were shown in opening window until recently. Now only 1 folder is shown. How do I get back to all accounts/folders shown display? I have tried restarting thunderbird and rebooting the computer. Thuderbird still opens in single folder format and I can't get to an all folders display. The all folders display flashes quickly when opening thunderbird before going to single folder format.

Found the problem. somehow I had closed the folders pane to zero or near zero width. Pulling it open with the mouse solved the problem. Looking for answers told me that others have likely also had this problem.

Note that it it impossible to navigate with the drop down menu. I tried. One can go to a folder or sub-folder once but not change folders or go to a folder from a sub-folder. Rebooting got me out of the sub-folder so I could verify my account. I don't know why the drop down menu is there if one can not use it.