After announcing plans for Project Quantum late last year, Mozilla has now launched its overhauled Firefox in beta stage, making it an increasingly imminent challenge to Chrome. And it's a very feasible challenge too, given the browser's improvements. Speed is its main selling point -- Mozilla says Quantum is roughly twice as fast as Firefox was a year ago. This is largely due to a new CSS engine written in Mozilla's programming language, Rust, which operates across cores rather than as a single memory-hungry process on one core. It also priorities tabs, loading up in-use tabs before background ones, which Mozilla says makes Quantum 30 percent more RAM-efficient than Chrome. The team has also tracked down and eliminated 469 bugs that were contributing to slowness.