User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:73.0) Gecko/20100101 Firefox/73.0
Steps to reproduce:
I created a new thunderbird profile. I changed the configuration to store messages with each message in its own file. I added a gmail account. I added a news and feeds rss account. I searched news feeds for unread articles and then saved the result as a search folder. I looked at the first unread article. I exited thunderbird and relaunched it.
The unread search folder does not appear.
There should be a folder called unread which gets populated with the unread news articles (status isn't read).
This has worked before but it maybe related to the setting to store emails each email in its own file, although I cannot see why.
find .thunderbird/ -print|grep -i unread