When connecting #Thunderbird to a new IMAP account that's full of folders, apparently it doesn't automatically "subscribe" to any except the inbox...
...and you have to manually choose all of them, one at a time. There's no way to "select all", and selecting a folder does not select any of the folders underneath that (even if the folder is collapsed).
You can't even use the keyboard to make the process go faster. The ⬇️ key does move the highlight down, but the [space] key doesn't select, and [enter] just submits what you've done so far.
On the internet, everyone knows you're a cat — and that's totally okay.