A couple of suggestions in this thread: https://support.mozilla.com/en-US/forum/1/662747?

~~green:cor-el~~'s suggestion applies to Menu Bar only. My suggestion applies to entire interface, not just Menu Bar and Toolbars

You can try to increase the String pref layout.css.devPixelsPerPx from 1.0 to 2.0 in 0.1 steps to see what works best. In Firefox 3.6 and later this pref is a String value parsed to a float and allows to fine tune the dimensions of all elements more precisely.

To open the about:config page, type about:config in the location (address) bar and press the Enter key, just like you type the url of a website to open a website. If you see a warning then you can confirm that you want to access that page.