uBlock Origin
Licensing: GNU General Public License (GPL) v3.0 (Source code)
uBlock Origin is NOT an "ad blocker": it is a wide-spectrum blocker -- which happens to be able to function as a mere "ad blocker". The default behavior of uBlock Origin when newly installed is to block ads, trackers and malware sites -- through EasyList, EasyPrivacy, Peter Lowe’s ad/tracking/malware servers, Online Malicious URL Blocklist, and uBlock Origin's own filter lists.
addons.mozilla.org for use with Firefox