EDIT: It seems that you can accomplish your goal according to the method described here. But instead of:

menubar { background-color: red; }

You should put (with 20 replaced by whatever font size you want):

* { font-size: 20pt !important }

But this might be too technical. I am sure jscher2000's answer will work a lot better for you.

Obviously there are still a lot of things I don't know about FF :)

ORIGINAL: Those fonts are controlled by your operating system. If you want bigger fonts in menus (not just those in firefox), you can try this.

Just a disclaimer. I use a different operating system, so I can't verify what is described in the link actually works.

There is a Firefox extension which allows you to enlarge the fonts in the menu and toolbar area. You could give this a try and see whether it works for bookmarks, too:

https://addons.mozilla.org/firefox/addon/theme-font-size-changer/

Its button usually appears toward the end of the main toolbar after you install it. Once you are done setting the font size you want, you can take the button off the bar using the Customize feature. See: Customize Firefox controls, buttons and toolbars.

Note that on Mac you likely can't modify the font size of the menu bar as that bar is under control of the Mac OS and can't be modified via an extension or code in userChrome.css. So the above mentioned Theme Font & Size Changer extension will only work for the Bookmarks Toolbar and possibly the bookmarks sidebar.

It worked