RedMorph for Firefox is a free ad blocker and content filter.

Install the extension and it seems to work much like any other ad blocker. No setup is required, you just browse as usual and the RedMorph icon displays the number of ads/ trackers/ etc it's blocked.

There's one small extra in the shape of a tiny RedMorph icon bottom-right of the page. If a page isn't working, hover your mouse cursor over this and you're able to choose an alternative protection level for the current site only.

By default RedMorph blocks all trackers and third-party content, but dropping down to Standard or Basic Protection allows more content and may get your page running again, or you're able to turn off RedMorph entirely. Whatever setting you choose will be applied whenever you revisit the current site, but your default settings will still work everywhere else.

RedMorph includes SpyderWeb, a graphical visualisation of the various domains your current site is trying to access. These might be subdomains of its own, maybe a CDN, but you might also find ad sites, trackers and more. This doesn't have any great practical value, but it's sometimes interesting to see how much a page is trying to do behind the scenes, and all this extra activity might also explain site performance issues.

RedMorph also has simple website blacklists and whitelists. You're able to prevent access to specific sites, or allow access to specific websites only.

A "Word Filter" enables checking page content for specific words. If any are found these can be masked (replaced with **), or you can have RedMorph block the site entirely.

Paying to upgrade also gets you an anonymous proxy and VPN service.