Firefox previews site-isolation tech in move to catch up to Chrome 'Project Fission' is part of a long-term effort by Mozilla to harden Firefox's defenses by isolating malicious sites and components. Mozilla has announced that a years-long effort to harden Firefox's defences can now be previewed in the browser's Nightly and Beta builds. Debuting as "Project Fission" in February 2019, the project was also linked to the more descriptive "site isolation," a defensive technology in which a browser devotes separate processes to each domain or even each website, and in some cases, assigns different processes to site components, such as iframes, so they are rendered separately from the process handling the overall site.