Comparing browsers peformance is not that simple, due to the immense complexity. The only part that's significantly faster in chrome is the JS engine (V8). The CSS engine, for example, is much faster in firefox (try stylebench if you don't believe me) - at least it was last time I tried it. Here are some benchmark results for comparison: https://arewefastyet.com/linux64/benchmarks/overview?numDays=60
Also, firefox uses much less resources (CPU/Ram), so it consumes less battery and doesn't slow down other programs as much while it's running in the background.