Shut down Firefox. Open terminal.
cd ~/Library/Application Support/Firefox/Profiles
open ~/Library/Application Support/Firefox/Profiles
Navigate to target profile.
E.g.,
cd nge0hgfo.default
Find potential places where blocked files are stored.
grep -rli 'client-cdn' .
./jetpack/[email protected]/simple-storage/store.json
./places.sqlite
Edit file and remove entry(ies).
mvim ./jetpack/[email protected]/simple-storage/store.json
{"blocked":["http://client-cdn.crystalcommerce.com/photo/blueoxgames/file/3f0456a0421a11e5952f357ffe9c16f7/logo.png"]}
Save and fire up FireFox again.
{"blocked":[]}