User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

1 Create folder named 'Social Media/Search' in Local Folders in Thunderbird

2 Create subfolders 'Facebook', 'Instagram', 'Google'

3 Rename folder to '.Social Media/Search'

Actual results:

1 'Social Media/Search' folder appears correctly under Local Folders in Thunderbird
Folder created in <profile>\Mail named 'Social Media/<hash1>.sbd'

2 'Facebook', 'Instagram', 'Google' folders appear in 'Social Media/Search' under Local Folders in Thunderbird and work as expected
Files named 'Facebook', 'Facebook.msf', 'Instagram', 'Instagram.msf', 'Google', 'Google.msf' created in <profile>\Mail\Social Media/<hash1>.sbd folder

3 'Social Media/Search' folder disappears entirely from Local Folders in Thunderbird and no new folder '.Social Media/Search' appears - subfolders and any messages contained appear completely lost [inducing minor/major panic!]
New <profile>\Mail\<hash2>.sbd folder created containing files 'Facebook.msf', 'Instagram.msf', 'Google.msf'
Original <profile>\Mail\Social Media/<hash1>.sbd folder remains and still contains files 'Facebook', 'Instagram', 'Google'

Expected results:

Either

error dialog "Folder name can't contain '/' " at step 1

Or / is allowed and hash is created correctly, folder can be renamed

TEMP SOLUTION TO RETRIEVE MESSAGES/FOLDERS
Close Thunderbird, move files from <profile>\Mail\xxx/<hash1> folder to <profile>/Mail/<hash2> folder, restart Thunderbird

Yes, slashes should be avoided, see meta-bug 124287.