It appears that the primary folders:
... are automatically subscribed to, and that there is no way to stop Roundcube from subscribing to them (i.e., I see no checkboxes next to those folders in Settings > Folders). I find this annoying because I don't use all of these folders (I really only use Inbox and Sent), and they show up when I check my mail in Thunderbird because they are subscribed to via Roundcube. I can unsubscribe the unwanted folders in Thunderbird, which temporarily solves the problem in Thunderbird, but the folders reappear whenever I check my mail again in Roundcube.
It's not the biggest deal in the world, but I'd like it if there was a built-in method to unsubscribe from all folders.