How to achieve high-quality downscaling of diagrams (like in Firefox)?

I often need to downscale raster images consisting of thick straight lines and other thick curves, like this one.

Using GIMP 2.8.16 and its various interpolation options to resize the image to 600×600 sq. pixels, I obtain the following images:

No interpolation

Linear

Cubic

Sinc (Lanczos3)

Now, compare this to the result achieved by Firefox the web browser:

Firefox 47.0

Look, for instance, at the topmost grey line. If you compare¹ each of the GIMP-produced images with the one obtained using Firefox carefully, you'll see that in each comparison, the line looks far better in the Firefox-generated image.

Now, as you might imagine, using a web browser to downscale images isn't very convenient, so I wonder if there is any image editor able to downscale images like this as nicely as Mozilla Firefox.

¹ Make sure your browser isn't applying any interpolation of its own when you view the images. You might want to open a new browser window and open two tabs in it: one with a GIMP version, and the other with the Firefox version. Then use Ctrl+Tab to switch tab quickly. This will make the differences very obvious to most human observers.

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