That's not true at all. Chromium is far more secure than Firefox.

I would say that's highly debatable.

Firefox lacks many security features that Chromium has such as site isolation,

They have this feature in nightly and it will be out to stable this summer.

a hardened memory allocator (mozjemalloc is just a few security features tacked on to jemalloc and is not similar to a hardened memory allocator),

This isn't really needed. I've never seen any evidence that Firefox is less secure due to this.

Control-Flow Integrity and many more.

Chrome literally just added this recently and Firefox is planning to follow suit and add it as well.

Firefox's current security features such as its browser sandbox are a lot worse than Chromium's.

No evidence is presented for this claim.

Excluding the issue of site isolation, only the Firefox sandbox on Windows is similar to Chromium's but even then, it lacks features such as win32k lockdown.

Also no evidence is presented for this claim. If you refer back to your own link to Mozilla's security sandbox, the difference between Windows and Linux sandbox is only Level 5 vs Level 4 respectively. And this is simply due to a pair of mitigations not implemented in Linux yet. So very little difference there.

The others are very insecure and the Linux sandbox can hardly be called a sandbox at all as there are many trivial escapes such as the X11 server (this is also a 5 year old, critical issue).

I would hardly call that a major issue. It seems like they are addressing it right now and it mainly centers around WebGL... which is the very first thing that should be disabled on any hardened Firefox setup.