Update: the subfolders do eventually appear 38 seconds after tapping the bookmarks folder. Prior to the most recent update it took approx 1 to 2 seconds for the subfolders to appear.

What caused it in version 148 is Bug 1988315 - Add/Edit bookmark, change storage location: With extensive bookmark collections, the folder list loads extremely slowly and it's going to be fixed in Bug 2009006 - Expanding folder with many items takes long time.

I'm still having this problem. Now I have a bunch of bookmarks in my main bookmark folder that I'm going to have to sort when I can find the time and have access to a desktop computer.

This, combined with the ongoing youtube functionality problems, has motivated me to give Opera a try. Unlike other mobile browsers it seems that lately Firefox has been having a hard time maintaining basic functionality.