I don't use IMAP in Thunderbird, so I can't provide step-by-step instructions, however, other e-mail programs have settings for whether or not to synchronize each individual folder on a schedule or using push IMAP (vs when clicked). If Thunderbird has those settings, I would expect them to appear in the folder properties or account settings. Perhaps this will be helpful, but I don't know if you've already done some digging like this or not.

FYI, a quick search brought me to this page: https://support.mozilla.org/en-US/kb/configuration-options-folders

One item listed in that page is this:

Configure IMAP folders, connections and namespaces

Menu location: Tools | Account Settings | <account name> | Server Settings | Advanced

Perhaps that would be a good place to start since it does mention IMAP folder connections, but folder properties still might be an easier place to peek first, just in case. For that, the site lists this:

Configure folder properties (IMAP folders only)

Menu location: Edit | Folder Properties | General Information

However, I think you could right-click on the "Bulk Mail" folder and hit properties in the pop-up menu.

Thanks! That solved my problem!

I do still wonder why the Junk / Spam / Bulk Mail etc. folders are not automatically selected as a folders for Thunderbird to look into, as new mail will (often by default) get into there as well as in inbox.

pekka