Same here in Firefox 31 in Seller Hub actives. Sits and spins, but never closes. If I hit save, and then close it via the X button or the cancel button it goes away, and does save the edits, but I have to refresh the active selling page for the changes to actually show.
In Firefox 52 it fails the same way.
The other completely useless POS that eBay forced me into
https://my.ebay.com/ws/eBayISAPI.dll?MyEbayBeta&CurrentPage=MyeBayNextSellingdoesn't have any option to edit the note (or do much of anything or work in my phone browser)
Just another part of eBay's changes that are making older browsers unusable and even apparently conflicting with the newest versions, and another brick in the wall.