Firefox has rolled out a new Site Isolation feature that greatly improves the security of the web browser by isolating every domain visited into a separate process.  This provides protection against Meltdown and Spectre style attacks and cross site attacks that attempt to thwart the Same-Origin policy.

Known internally as Project Fission, this significant development has been in the works since 2019 and required a major rearchitecting of the browser to change the design from ‘one process per tab’ to ‘many processes per tab.’  Now, Firefox will spawn a new process for each web domain that is present within the page being rendered in the tab.  This means each iFrame or other embedded content such as a script delivering a Facebook Like button or advert will run in a discrete operating system process with its own isolated memory space.  According to the Firefox team, these changes will also improve browser performance and stability.

Google Chrome rolled out its version of site isolation back in Chrome 67 whereas Safari does not currently provide this level of site isolation.

How to activate Site Isolation mode in Firefox

Site Isolation mode in Firefox is not yet enabled by default, but you can turn it on with these steps:

1. Navigate to about:config in the Firefox address bar
2. Set fission.autostart pref to true
3. Restart Firefox