I love SS and have used it for years on two Windows 7 pc's. Now I have installed Thunderbird and SS on a new Windows 10 pc. All is fine, never had a problem with it, except when I exported TB from one computer to another without updating the signature files path in the new computer. The signatures appeared 'grey' (non-selectable) when composing a new message. It was my fault. Updating the file path everything went fine.
This review is for a previous version of the add-on (1.7).