Firefox

As a security researcher, running each site in its own process isn't enough. Chrome has a much stronger multiprocessing model on most platforms. For example, Chrome on Android sandboxes between processes whereas Firefox simply relies on the built-in Android sandbox, which provides limited protection between these processes. It's much easier to break out of the sandbox in Firefox because it's easier to move laterally, for one. Those processes have to communicate with each other at some point.

But, don't believe me just because I claim any sort of credential on the Internet. It's such a difference in security that GrapheneOS strongly discourages using Firefox for its weak implementation in addition to the link I provided above. From the link:

Worst of all, Firefox does not have internal sandboxing on Android. This is despite the fact that Chromium semantic sandbox layer on Android is implemented via the OS isolatedProcess feature, which is a very easy to use boolean property for app service processes to provide strong isolation with only the ability to communicate with the app running them via the standard service API. Even in the desktop version, Firefox's sandbox is still substantially weaker (especially on Linux) and lacks full support for isolating sites from each other rather than only containing content as a whole.

I love Firefox. I use it anyway. It's not insecure. But it's absolutely not as secure because it lacks modern exploit mitigations. Running process per site is an improvement but it's still less secure than the architecture used in Chrome.

EDIT: Sound less entitled.