Great idea, poor execution Rated 2 out of 5 stars
Unfortunately, there's no diagnostic information provided, and the support site is really no help. The validator is not working for me, and reports that no sites are secured. That's odd, because http://test.dnssec-or-not.org/ reports I'm using DNSSec, and 'dig nic.cz. SOA +dnssec | grep "flags.* ad"' produces ";; flags: qr rd ra ad" on an alternate (Linux) machine. (Testing on Windows.) My local resolver is set up for DNSSEC operation, and seems to work correctly for all other cases.
What DNS resolver is set in preferences?
did you point DNSSEC Validator at your resolver (127.0.0.1 I presume) in preferences? If it still doesn't work, email me at ondrej.mikle at_sign nic.cz.
DNSSEC Validator will soon-ish get an overhaul and libunbound will be used instead of lowlevel libldns which should make results much better.
EDIT: there is option "extensions.dnssec.debugoutput" in about:config that enables debug output to console (I made a ticket for myself to add it later to dnssec-validator.cz text). Double-click it to set it to "true" and run firefox with console (e.g. terminal in Linux or -console option in Windows).