The issue is related to how Firefox handles site-isolated processes for documents loaded from data: URLs that result from redirects. Previously, Firefox would load such documents in the same process as the site that issued the redirect, bypassing site-isolation protections against Spectre-like attacks on sites hosting an "open redirect". Firefox has been updated to no longer follow HTTP redirects to data: URLs, addressing this issue. The vulnerability can be exploited by a remote attacker to bypass security restrictions and redirect a user to an arbitrary URL.