Sorry, Firefox Quantum is not compatible with Vista. The last version of Firefox to be compatible with Windows Vista is Firefox 52 ESR, which will continue to get security backports/updates until May 2018. In any case you should go about updating your operating system, or purchasing a new computer.

Thanks - can't afford to upgrade or get a new computer right now, hence the question.

You may as well give Linux a whirl. It's better than staying on an outdated Windows system plus you'll get the latest Firefox version.