This looks great, people need to be able to find out downloaded files checksums without resorting to command line tools!
I emailed the developers through the website too but thought I'd post here. I work on metalink which includes checksums & signatures in an XML file about downloads (also mirrors & other info).
DownThemAll is another extension that already supports metalinks, & could do the downloading. Could DTA make use of FireGPG if it is installed?