Subdomains shouldn't be a problem unless your base domain is in the Public Suffix List. According to MDN: > More specifically, Firefox double-keys all client-

Subdomains shouldn't be a problem unless your base domain is in the Public Suffix List. According to MDN: > More specifically, Firefox double-keys all client-side state by the origin of the resource being loaded and by the top-level site. [1] They linked the definition of a "site" to the HTML5 spec, which says this: > To obtain a site, given an origin origin, run these steps: [2] > 1. If origin is an opaque origin, then return origin. > 2. If origin's host's registrable domain is null, then return (origin's scheme, origin's host). > 3. Return (origin's scheme, origin's host's registrable domain). The HTML5 spec refers to the site's registrable domain according to the URL spec: > A host’s registrable domain is a domain formed by the most specific public suffix, along with the domain label immediately preceding it, if any. [3] Public Suffixes are defined according to a database that you have to explicitly register in [4]. If you aren't sure whether your base domain is registered as a public suffix, then it probably isn't. [1]: https://developer.mozilla.org/en-US/docs/Web/Privacy/State_Partitioning https://developer.mozilla.org/en-US/docs/Web/Privacy/State_P... [2]: https://html.spec.whatwg.org/multipage/origin.html#site https://html.spec.whatwg.org/multipage/origin.html#site [3]: https://url.spec.whatwg.org/#host-registrable-domain https://url.spec.whatwg.org/#host-registrable-domain [4]: https://publicsuffix.org/ https://publicsuffix.org/