That already exists in your browser: "Save image as..." or "Save link as..."
Please don't make AMO reviews to request features or talk about bugs. Send me an email or post an issue on gitlab. The relevant links are on the addon's main page here.
You don't. Downloads will and should go into the user's downloads folder. Please don't make reviews just to ask a question, that's what an issue tracker is for ("support site").
This addon does exactly what I want to save the images on the website by hover the mouse over the images. But there are some unuse features which could be removed for slim down the codes, unmainternance codes?
Don't worry about code maintenance, that's my job. The disabled features are there because they carried over from the original addon and the chrome version, and I expect them to be possible in the new firefox soon.