View > Toolbars > Customize > [ ] Use Small Icons

• Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/162063/

You can also increase the String pref layout.css.devPixelsPerPx from 1.0 to 2.0 in 0.1 steps to see what works best for you.
In current Firefox versions this pref is a String value parsed to a float and allows to fine tune the dimensions of all elements (user interface and web pages) more precisely.

If you need to increase (or decrease) the font size on websites then look at the NoSquint or Default FullZoom Level extension.

• Default FullZoom Level - https://addons.mozilla.org/firefox/addon/6965
• NoSquint - https://addons.mozilla.org/firefox/addon/2592

When I use the first add-on it increases the size of the Menu bar (etc.) but NOT the icons on the bar--which I have problems distinguishing one from another without dragging my cursor over each icon for the text identification.

Whoa! In customizing the header I selected Use small icons, then de-selected the use small icons. Walla! The icons are now enlarged.

Thanks! You are saving my tired old eyes!--cll

You're welcome.