Building a modern browser takes a large team. For example, there is a team on each of the following: * Layout (HTML, CSS) * JavaScript * Graphics: HTML, 2D c

Building a modern browser takes a large team. For example, there is a team on each of the following: * Layout (HTML, CSS) * JavaScript * Graphics: HTML, 2D canvas, 3D WebGL * Platform integration (windows, os x, gtk, etc.) * Audio * New web APIs * Security * QA * Release engineering * Testing * Optimization / low-level tooling * And others I can't think of right now. All major browsers have teams of hundreds of people or more. The other browsers than Firefox probably have larger ones, in fact, since their budgets are much larger (for example, just Chrome's ad budget was larger than Firefox's entire budget for everything). As for why multi-process took a while, it was working several years ago already and shipped in mobile firefox, but the hard part for desktop specifically was addons. That took several years, since it's both a technical issue and an ecosystem issue.