What’s the source for that claim? To my understanding, Firefox first got sandboxed processes for sites in 2021, and only recently this year got features to sandbox the GPU processes as well - playing catch-up by many years to Chrome, and exposing attack vectors for sites to gain access to OS-level API’s to meanwhile. And to my understanding, neither are enabled by default on Firefox for Android, because of ongoing compatibility issues for years bugzilla.mozilla.org/show_bug.cgi?id=1610822
My take is that Firefox or its’ derivatives are better for privacy, while Chromium is better for security, due to the vastly greater development resources.