User interface is mildly annoying. Every time I save a screenshot the add-on opens yet another instance of my system's (Xubuntu) file manager (in my case, Thunar). I'm amazed you managed to access the local filesystem from WebExtensions, though. How did you manage that? I can't find anything in the WebExtensions documentation that says that's supported. Is this add-on open source? I could use some file saving capabilities for my own add-ons.