Q: How can I collapse all folders?

Exactly what do you mean when you say collapse open folders?

Thanks for the clarification.

AFAIK that is not possible.

Let's say you have a folder called "Months" and inside Months there are 12 folders named with the months of the year.

Go to list view and top level folder (Months) press alt and right cursor key.

This will collapse "Months" showing the files and sub-folders enclosed.