Maybe twirling the top-level arrow, next to the account name, is the best option, since presumably it will force all subdirectories to be displayed (I didn't try that).

I mention this mostly to assert that the depth of the directory whose 'arrow is twirled' seems not to matter, as long as the twirled directory is a parent of the directory that won't appear.

The structure was Inbox -> Subdirectory 1 -> Subdirectory 2, where Subdirectory 2 represents the IMAP directory that was just added. It's curious to note that the new IMAP directory in question was 'two levels deep', too. As soon as I did that, the new directory appeared! nothing forced the new directory to be displayed - until I twirled-up and twirled-down the little arrow immediately to the left of the Inbox (in the folder pane panel). I had quit and relaunched Thunderbird, double-checked that I had subscribed to the IMAP folder in question, subscribed/unsubscribed, quit and relaunched again. What actually made the difference was collapsing and expanding the tree. I'm amazed that the answer is exactly as Randy Orrison described and Oliver Salzburg confirmed, above, over 3.5 years ago: How can I force Thunderbird to reload IMAP folder structureĮvery time I tried it, I was already subscribed to the new folder. (Somewhat) surprisingly, in 2015, with Thunderbird 38.2.0, forcing Thunderbird to refresh the IMAP folder tree after adding a new directory/folder is still necessary.

Thunderbird gmail