I'm still in a journey to find a way to make firefox queue download well. Just that and nothing else. This does that, however the "what to do when file already exist" only gave me options to replace or rename. No options to "ask me what to do" or "don't download".

Actually I don't even know if the queue worked. I set simultaneous download on specific domain to only 1 at a time but it still downloaded all of the image at once. Like 4 of them.