Sometimes RAM is cheaper than engineer salaries. Sometimes it isn't. It depends on problem domain, scale, and what you're tying to optimize for. Debating this just seems pointless.

In any case, the number one evaluation criterion for browsers is not the RAM profile.

If performance matters that much, perhaps you should instead be asking why we are writing applications and UI in an ugly evolution of SGML.

Before you fault the Moz devs, ask yourself why we have CSS for high DPI images and why designers embed videos into website headers. Maybe your needs aren't at all times reflective of the majority. The browser serves the spec. The spec serves every case you care to imagine. Because that's what we evolved a doc format to do.

(Just imagine a parallel universe where it was instead MS Word that evolved into a facebooking client!)