It's a little weird in Firefox 63 in Linux. Holding down the mouse drags the image, but after I release, it changes the curve correctly until my mouse leaves the image. It also works fine if I hold down middle click.

This can be fixed by inserting this line after line 308 of euler_explorer.js, in the onpointerdown handler:

    e.preventDefault();

By default, if you drag an image, then it moves the image. You should need to call preventDefault to prevent that, so I'm kind of annoyed with Chromium.