It can happen, at times, that single items, or one or more folders of items, get placed below the standard main folders: My Inventory and Library. Attempts to move or delete these items/folders result in a crash. This is a known server issue which goes back a few years at least.
Firestorm brings a way to repair this. Right click on the folder and you should find an entry “Move to Lost & Found”. Click that, and the folder will be moved to your Lost And Found folder. From there, you may drag it to where it belongs.
IMPORTANT This requires Firestorm 4.4.0 or later.