Mozilla Firefox - Allow unverified add-ons extensions plugins

Mozilla Firefox seems to have added a safety feature that prevents installing unverified add-ons, like Imagegrabber:
http://imagehost-grabber.org/index.php?topic=3457.0

When you try to install it you'll get a warning and then you are denied to install it.

To allow unverified add-ons, type about:config into the address bar, find xpinstall.signatures.required and set it to false.

Then you can install unverified, add-ons, I could grab an .xpi file from the desktop, drop it onto Firefox and then install.

More info:
https://support.mozilla.org/en-US/questions/1078339