It is annoying when you wish to save an image from a Facebook post and this will not work. But this is actually very easy using Firefox. Just right click on the image and select Inspect element with Firebug.
Then drill down through the code until the URL to the image is found, just select all this code and then CTRL-C it.
This is what it should look like. Open this in a new tab and then save the image. This should also work with Chrome if you select Inspect element.
That is how easy it is to do this.