Modern addons are not able to control download locations, sorry. This is a deliberate design regarding user settings in browsers. I couldn't implement it even if I wanted to.

Thanks for the constructive input, but unfortunately filename extensions are not a reliable way to determine file types. They can be missing, simply incorrect (how often I've come across a .jpg that was actually a png and vice versa...), or some website-specific filename thing that happens to have a period in it so it looks like an extension. Browsers can determine the content type themselves by looking at the data, but in HTTP requests the convention is that the server must provide a human-friendly filename with a convenient extension and only the browser can make a last ditch attempt. If a server provides no meaningful name for a file triggered by this addon, then there's nothing it can do to help the situation, it's the least qualified party involved.

I'm glad you like it, thanks for the message. "automatically assign an extension of your or the user's choice to any picture" this is possible, but it would be incorrect and bad behavior. Blindly defaulting to adding .jpg for example to any unknown image, means e.g. a png image could get the jpg extension. Nowadays this should cause no issues, but I consider it an anti-feature because it's simply wrong, so I will not implement it, sorry. Browsers have tricks to determine an extension and will do so whenever possible. If one cannot be determined by the browser, addons are not in a better position to try it. As for the AMO link, I'm surprised firefox doesn't provide that link by itself... So I guess, I might add AMO links to the descriptions next time I work on the text.

Thanks for the detailed feedback. As for file extensions, extensions are not actually a required thing in filenames, they're more of a loose convention. Websites tend to forego them depending on how their images are loaded (e.g. from data streams rather than files). It's the browser's responsibility to determine an extension if at all possible, based on data from the webserver if provided (such as a suggested filename, with or without extension). It's really not within an addon's power to ensure the presence of file extensions, or infer any, sorry.

Only via your browser settings. Addons are not allowed to manipulate download locations.

Sorry for the late reply, glad you like it, thanks. I like your idea, please follow progress here: https://gitlab.com/Marnes/doubleclick-image-downloader/issues/71

Glad you like it!

Notifications are controlled by firefox, addons can't change their timing, sorry. If the notifications bother you, you can turn them off.