Looks like you can add custom domains in about:config as a true boolean with browser.fixup.domainwhitelist.yourdomain.yoursuffix. Just needs to be full domain with tld.

https://bugzilla.mozilla.org/show_bug.cgi?id=1634650

https://support.mozilla.org/en-US/questions/1285922

Web on desktop with uBlock, Apollo on iOS. Third-party apps are being effectively banned soon though as they want $20 million per year from third-party app developers.