You can change the displayed font size in firefox

If the text is too small to read or takes up too much of a page you're viewing firefox has a builtin zoom:
ctrl + will enlarge the page (hold down control key hit plus sign)
ctrl - will shrink the page (hold down control key hit minus sign)
Or use the Scroll wheel

ctrl scroll up will enlarge the page (hold down control key move the scroll wheel up)

ctrl scroll down will shrink the page (hold down control key move the scroll wheel down)

ctrl 0 resets it to default  (hld down control key hit 0)

can be done multiple times, each time will increase/decrease the font size more.