[nsvd]

You should be able to change this in about:config no?

I use Ctrl-s often to save images.

[pennomi]

To my knowledge it is impossible to change this behavior in Firefox. You can hackily override it by adding an extension which injects some js that captures the ctrl-s keys, but that can also break sites that legitimately use that functionality.

Data files like images are a reasonable use of the shortcut. Downloading HTML is nearly always unintended behavior.