As I learned recently , modern versions of Firefox have two different techniques that try to defeat (unknown) tracking cookies. As covered in the browser addon JavaScript API documentation, in Tracking protection , these are called first-party isolation and dynamic partitioning (or storage partitioning , the documentation seems to use both). Of these two, first party isolation is the easier to describe and understand. To quote the documentation:

When first-party isolation is on, cookies are qualified by the domain of the original page the user visited (essentially, the domain shown to the user in the URL bar, also known as the "first-party domain").

(In practice, this appears to be the top level domain of the site, not necessarily the site's domain itself. For example, Cookie Manager reports that a cookie set from '<...>.cs.toronto.edu' has the first party domain 'toronto.edu'.)

Storage partitioning is harder to understand, and again I'll quote the Storage partitioning section of the cookie API documentation:

When using dynamic partitioning , Firefox partitions the storage accessible to JavaScript APIs by top-level site while providing appropriate access to unpartitioned storage to enable common use cases. [...]

Generally, top-level documents are in unpartitioned storage, while third-party iframes are in partitioned storage. If a partition key cannot be determined, the default (unpartitioned storage) is used. [...]

If you read non-technical writeups like Firefox rolling out Total Cookie Protection (from 2022), it certainly sounds like they're describing first-party isolation. However, if you check things like Status of partitioning in Firefox and the cookies API documentation on first-party isolation , as far as I can tell what Firefox actually normally uses for "Total Cookie Protection" is storage partitioning.

Based on what I can decode from the two descriptions and from the fact that Tor Browser defaults to first-party isolation, it appears that first-party isolation is better and stricter than storage partitioning. Presumably it also causes problems on more websites, enough so that Firefox either no longer uses it for Total Cookie Protection or never did, despite their description sounding like first-party isolation.

(So far I haven't run into any issues with first-party isolation in my cookie-heavy browser environment . It's possible that websites have switched how they do things to avoid problems.)

First-party isolation can be enabled in about:config by setting privacy.firstparty.isolate to true. If and when you do this, the normal Settings → Privacy and Security will show a warning banner at the top to the effect of:

You are using First Party Isolation (FPI), which overrides some of Firefox’s cookie settings.

All of this is relevant to me because one of my add-ons, Cookie AutoDelete , probably works with first-party isolation but almost certainly doesn't work with storage isolation (ie, it will fail to delete some cookies under storage isolation, although I believe it can still delete unpartitioned cookies). Given what I've learned, I'm likely to turn on first-party isolation in my main browser environment soon.

If Cookie Manager is reporting correct information to me, it's possible to have cookies that are both first-party isolated and partitioned; the one I've seen so far is from Youtube. Cookie Manager can't seem to remove these cookies. Based on what I've read about (storage or dynamic) partitioned cookies, I suspect that these are created by embedded iframes.

(Turning on or off first-party isolation effectively drops all of the cookies you currently have, so it's probably best to do it when you restart your browser.)