Workaround Rated 4 out of 5 stars
I have at least got a work around for this nice add-on.
Copy what you want to save to the clipboard with the copy option. Then use some image editor such as Gimp (Linux) paste the image into a new one and save. A bit cumbesome, but from all the screenshot programs available it is abou the onlt one which works in this way wit FF18.
All the screen shot add-ons have similar problems at this stage, they do not save to disk...
FF 18 dies not save to disk Rated 3 out of 5 stars
In FF 18 this addon does not save to your local disk, however it does copy to the clipboard from where you can get the image..