This will enlarge both the toolbar area and the content area:
In a new tab, type or paste about:config in the address bar and press Enter. Click the button accepting the risk.
In the filter box, type or paste pix and pause while the list is filtered
Double-click layout.css.devPixelsPerPx and change its value to 1.25 and click OK which corresponds to 125% font size in Windows display settings.
The toolbar area should enlarge after a few moments. Too small? Try 1.5 and see whether that suits you better. And so on. If you are on Mac and use a Retina display, anything below 2.0 may be smaller than what you started with, so start with 2.0.