RequestPolicy is a browser extension for Firefox(*) and SeaMonkey that gives you control over cross-site requests. It does this through user-defined whitelists and blacklists. Users can also subscribe to policies (whitelists and blacklists) maintained by others.
Have a look at our wiki, especially the Quick Start guide and the FAQ.
(*) Firefox 57+ is not yet supported
Almost all code is licensed under GPL v3 or later, some files are licensed under MPL 2.0. See LICENSE.