To have all IMAP folders checked for new mail, I think you need to double-click (toggle) the preference mail.server.default.check_all_folders_for_new to true in Tools/Options/Advanced/General/Config. editor.

http://kb.mozillazine.org/How_do_I_check_for_new_messages_in_other_folders

Unfortunately the preference setting in the Config Editor does not alter the default setting of the "When getting new messages for this account, always check this folder" property in new folders, and so far as I can gather from the mozillazine article sfhowes refers to, it overides the folder property.

Whilst I usually want to always check this folder for new folders, at least for a few days until all messages are synchronised on both my machines, I do not want to always check every folder (including junk, and some which hold historic information, which will rarely if ever be added to) every time I get new messages for the account because of the possible (likely?) performance effect referred to in the mozillazine article, particularly when loading Thunderbird.

The FolderCheck addon also referred to in the mozillazine article would be very helpful in enabling me to quickly alter the "When getting new messages for this account, always check this folder" property in folders, but unfortunately it has not been updated for the latest version of Thunderbird.

Surely the default folder properties for new folders must be set somewhere, or are they hard coded?

"sfhowes

To have all IMAP folders checked for new mail, I think you need to double-click (toggle) the preference mail.server.default.check_all_folders_for_new to true in Tools/Options/Advanced/General/Config. editor. "

This is helpful wish i knew when i set up all of my accounts a few months ago instead of going thru one by one. The accounts I use get nearly no junk mail so I do sync all folders to capture legit mail that gets sorted to spam.

OP - sounds like you want to change defaults without changing defaults? For folders you add only? Seems like a simple checking the box on a case by case basis may be your best option?