Firstly, sandboxing is a part of layers of layers of security mechanisms. You cannot tell the overall security by just looking on it. In recent years, a lot has changed in Firefox' sandbox. I don't know if it cached up, but I'd guess it has become quite a good sandbox by now.
What is more important for me, however, is the discrepancy in zero days. Chromium just had more in the past. That may be due to the gap on market share and therefore interest of "hackers" but it shows that you can't just claim that Firefox "sucks ass" and Chromium is safer.
I think there is just a lot of opinion in the internet regarding this topic while not a lot of information.
E: my intention was not to attack you. Security is a complex topic. I just couldn't verify your claim that Chromium would be "undoubtedly" better.
I imagine they are THAT scared of their husbands that they want to double check a fact they already know. So yes, deeply concerning...