Firefox continues cracking down on tracking with cache partitioning

Firefox version 85 will be released in January 2021, and one of its features is increased user privacy via improvements in client-side storage (cache) partitioning. This has been widely and incorrectly reported elsewhere as network partitioning, likely due to confusion around the privacy.partition.network_state flag in Firefox, which allows advanced users to enable or disable cache partitioning as desired.

In a nutshell, cache partitioning is the process of keeping separate cache pools for separate websites, based on the site requesting the resources loaded, rather than simply on the site providing the resources.

With a traditional, globally scoped browser cache, you might see behavior like this:

1.     user browses to https://coolwebsite.com/
2.     many different resources are loaded and cached, including https://coolwebsite.com/logo.jpg
3.     user browses to https://shadywebsite.com/
4.     in a hidden div, shadywebsite loads https://coolwebsite.com/logo.jpg
5.     shadywebsite uses JavaScript elements to time how long the user's browser needs to render logo.jpg
6.     Since https://coolwebsite.com/logo.jpg was in cache, it renders in under five milliseconds
7.     shadywebsite now knows that the user has recently visited https://coolwebsite.com/—because if logo.jpg hadn't been cached, it would have taken longer to render in-browser.