More from the article ...

... A malicious website can create a large file in the browser's private storage. It can then repeatedly read from this file to check for small changes in the time it takes to access it. These timing changes occur because other programs and browser tabs are also using the SSD, creating competition for drive access, per Ars Technica.

By tracking these timing patterns and feeding them into a trained convolutional neural network, the attacker can determine which other websites and apps are open on the device. This works in different web browsers and can recognize common applications based on their input and output patterns. FROST does not read actual files or decrypt HTTPS traffic, but it can reveal what services run in the background. ...

imo, browser JavaScript is evil.