Chromium is far more secure than Firefox. Firefox's sandboxing and exploit mitigations are much poorer than Chromium's. This article is not blindly hating on Firefox but is a factual analysis of its weaknesses.
Sandboxing is a technique used to isolate certain programs to prevent a vulnerability in them from compromising
the rest of the system. All common browsers nowadays include a sandbox. The browser splits itself up into different
processes (e.g. the content process, GPU process, etc.) and sandboxes them individually. It is very important
that a browser uses a sandbox. Otherwise, any exploit in the browser can be used to take over the rest of the system.
With a sandbox, they would need to chain their exploit with an additional sandbox escape vulnerability.
However, sandboxes are not black and white. Just having a sandbox doesn't do much if it's full of holes. Firefox's sandbox is quite weak for the following reasons:
isolatedProcessfeature along with a more strict seccomp-bpf filter.
This is a non-exhaustive list — the above issues are only a few examples.
Exploit mitigations eliminate entire classes of common vulnerabilities / exploit techniques to prevent or severely hinder exploitation. Firefox lacks many important mitigations while Chromium generally excels in this area.
Firefox does have some parts written in Rust, a memory-safe language,
but the majority of the browser is still written in memory-unsafe languages and the parts that are memory-safe do not include important attack
surfaces so this isn't anything substantial and Chromium is working on
switching to memory-safe languages too.
Additionally, writing parts in a memory-safe language does not necessarily improve security and may even degrade security by allowing for bypasses of exploit mitigations. Some security features are geared towards a particular language and in an environment where different languages are mixed, those features may be bypassed by abusing the other language. For example, when mixing C and Rust code in the same binary with CFI enabled, the integrity of the control flow will be guaranteed in the C code but the Rust code will remain unchanged because buffer overflows are impossible in Rust anyway. However, this allows an attacker to bypass CFI by exploiting a buffer overflow in the C code and then abusing the lack of protection in the Rust code to hijack the control flow. Mixed binaries can be secure but only if those security features are applied for all languages. Currently, compilers generally don't support this, excluding Windows' Control Flow Guard support in Clang.
Firefox also uses RLBox but this is only used to sandbox a single library, Graphite and again, is not anything substantial.
Many security experts also share these views about Firefox.