On the Firefox menu bar, click "Tools" then "Options" Then "Content". Under the "Content" option, there are drop-down menus where you can change both the font and the font size. Change both to whatever is best for you. If the menu bar isn't visible, right-click just above the address bar and tick the box for it.
Windows 10 Home vs Windows 10 Pro comparison: Here are the extra features you get in the Pro version