Money isn't necessarily the issue for Mozilla, they managed to create a decent browser with a fraction of the big player's budget. As always, it's the triangle of complexity and time and manpower, they just can't fight on all fronts at once. At the moment, they are trying to get rid of a lot of legacy that prevented them from improving some aspects of the system (see project e10s, WebExtensions, etc.) all while preserving as much of the ecosystem as possible – which is very hard due to the current extension system that was created more than 10 years ago and just doesn't fit the bill anymore. E.g. e10s is already going on for 5 years or so and it just started rolling out a few months ago.

Good news is that with these projects completing (improving overall responsiveness), there's a huge new project starting (Project Quantum) that aims to overhaul major parts of the rendering engine. This will integrate a lot of the research and investment poured into Rust and Servo, which will probably outrun current browsers by an order of magnitude (at least in some aspects that has already been shown, see WebRender).

As opposed to Google, Apple and Microsoft?

Consider what happened to Opera. I think you vastly underestimate the complexity of modern browsers. By orders of magnitude.