If a user tests Chrome and Firefox and only Firefox has bad performance/battery life, they'll blame Firefox.
Mozilla is aware - there are a few bugzilla reports open about this - but like most Firefox bugs it takes months or years to be fixed. Last year (nov/dec?) they fixed something that improved battery life, but it's still worse than Chrome for me.
Mozilla has been tracking cpu utilization/power consumption issues with certain MacBooks since Quantum was released. It seems to mostly affect MacBook Retinas without discrete GPUs. From the big tracker it appears the latest theory is it caused by an interaction between Firefox's compositor and the macOS compositor.
Mozilla is aware - there are a few bugzilla reports open about this - but like most Firefox bugs it takes months or years to be fixed. Last year (nov/dec?) they fixed something that improved battery life, but it's still worse than Chrome for me.