There's just two things that would need improvement:

1. When I click the Firefox icon top left on this page and choose "Save image as...", it will suggest "firefox.png" as filename. When I use this add-on on the image, it is saved as "firefox.png?b=95020746-59df8772" which the system will not recognize as PNG.

2. The menu entry is named "Save Image to Downloads...", but "..." indicates, that this action requires further user interaction and usually opens a dialog. This is exactly what this add-on is not doing (it shows no dialog, it does not require further user interaction), so there should not be "..." at the end.