How to change save image to file default name?

I have a canvas HTML5 tag that is created by Caman.js.

When i click right in FF and save to file the default name for the file is canvas.png. Since i create a lots of files and need to save them this is unfortunate, because i need to set different name for each.

What i would like to acomplish is that when saving first image save to file dialog shows firstfile.png and second secondfile.png and so on. So i don't need to change name in the save dialog window.

How to change the default name of save to file dialog in FF using HTML and JS?