While the extensions can be downloaded from the browsers’ respective stores, Windows Defender for Chrome and Firefox won’t work without help from Edge. If it determines that the URL is not in the trusted list, it will open the website in an isolated Edge session. However, any link clicked in the isolated session will still launch in the original browser if it’s a trusted website.
To use the feature, both the Windows Defender companion application from the Microsoft Store and the browser-specific extension must be installed. Since it’s still in preview mode, only Insiders can access it at the moment, though it will eventually have a wide release once it’s been tested more thoroughly.