Consider this, Arch users only update their browser when they manually update their system. If you do this daily then you won't be behind too much, but less diligent Arch users might use an outdated browser for days or even a week or longer. Even the daily Arch updater might miss the Firefox update by a few minutes and spend the whole day using an outdated browser, assuming they update when they boot their system. Meanwhile security updates for browsers are pushed automatically on other distros.