There are features Chrome has that Firefox doesn't. There are also features that Firefox has that Chrome doesn't, such as Stylo and WebRender.

You can argue that Site Isolation is more important than large performance improvements (I think that's a losing argument, given that Site Isolation as presently deployed in Chrome doesn't even defend against compromised renderers and as such is restricted to being another Spectre mitigation), but you're being way too glib here.