Look into what Zypak is. It’s the sandbox that Chromium/Electron uses.
Flatpak has its own sandboxing but as a result it does disable the browser’s default sandboxing method, which is arguably much more tested and secure to an extent. Chromium and Firefox do not contain the same sandbox methodology by the way.
Personally, I am not too worried about this and feel there are other larger security risks for the average user that should be addressed upstream in the Linux desktop. If you would prefer the Zypak sandboxing that the browser comes with then I recommend installing Chromium in a Distrobox container. I don’t think anything will change and the default will be Flatpak Firefox for the 3 end-user images.