do not have mouse
How exactly do you use a PC without a mouse! Just go buy any mouse at Futureshop for $10
hold control and scroll up on your mouse wheel
hold control and press + / - to zoom text in / out
do not have mouse
He probably uses a laptop with a touch pad for a mouse or is using a mobile device (smart phone, tablet, etc).
You can go into your preferences of Firefox and set your default font and text size.
Cheers