The public suffix list is a database of effective top-level domains (eTLD), which are the public suffix of URLs. This database is included in browsers (at least by Firefox, Chrome and Safari - sources below) and may be updated remotely. There have been feature requests for an API that allows extensions to identify the public suffix (eTLD) of a given URL:

• Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1315558
• Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1163295
• (rejected) request for a web platform API: Public Suffix API whatwg/url#528

There are solve known problems with the application of public suffix list (https://github.com/sleevi/psl-problems), but that does not necessarily rule out an extension API with such access. Extensions that need to rely on the public suffix list currently need to rely on alternatives, such as bundling the database with the extension, at the risk of having incompatible interpretations of the "public suffix of a URL" between the browser and the extension. With proper documentation of the problems associated with the public suffix list, extension authors can make a conscious decision to use the API when they need to.

This issue is to track use cases and the desired shape of the API. For example, the following would be the minimum:

let suffix = await browser.publicsuffix.getPublicSuffix("www.example.co.uk");
// Result: co.uk

Here are other examples of APIs to query the public suffix:

• https://searchfox.org/mozilla-central/rev/49566d906ad040cf233b0a5acd597e1a63b92f72/netwerk/dns/nsIEffectiveTLDService.idl
• https://source.chromium.org/chromium/chromium/src/+/main:net/base/registry_controlled_domains/registry_controlled_domain.h;drc=4f516be19f2c1d35dc3240d050d84d10f0d6f726
• https://github.com/WebKit/WebKit/blob/0a51cee7874970c895c648c0b664b511ecf72ed5/Source/WebCore/platform/PublicSuffix.h