(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.
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 structure
Every time I tried it, I was already subscribed to the new folder. So that wasn't the problem. What actually made the difference was collapsing and expanding the tree. Thanks ;)
I had quit and relaunched Thunderbird, double-checked that I had subscribed to the IMAP folder in question, subscribed/unsubscribed, quit and relaunched again... 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). As soon as I did that, the new directory appeared!
It's curious to note that the new IMAP directory in question was "two levels deep", too. The structure was
Inbox -> Subdirectory 1 -> Subdirectory 2, where
Subdirectory 2 represents the IMAP directory that was just added. 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. 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).
Thanks to everyone who contributed to this most frustrating problem's resolution! Full credit to Randy Orrison for the original solution.
Who's game for opening a bug report (or citing an existing report)?