I've seen it posted across several sites and forums as a request, seeking help or a frustration, and it all comes back to having to do some manual fiddling around with .css files the 'average' user may have no intention of fooling around with -
For consideration for smaller-screened devices (like laptops) and / or folks whose vision may not be perfect, the ability within Preferences to create a larger Bookmarks Bar and increase the size of the font of items in it accordingly. Without using Zoom to enlarge everything - web pages, etc.
Heck, in my case (running Firefox on a variety of Macs - MacBooks and Desktop), I'd be happy if the font size could be scaled up to that of that found in the controls in the Menu Bar. Others might want larger...
How could this have been missed (or possibly removed for some arcane reason) for so long?