Allow for Bookmarks toolbar font size customization (or auto-sizing)

[nonlinearize]

1. In Firefox, the Bookmarks toolbar appears to have one default font/icon size, and it's rather small and unattractive.

By contrast, Chrome's default is a larger and more attractive display.

Ideally Firefox would allow users to customize font size (or implement an option for auto-sizing, so that a certain number of toolbar bookmarks would fill the full toolbar width).

*This is another place where with added customization options, Firefox could easily be more competitive with other browsers.*

2. Notice too that Chrome allows users to customize the home page image, whereas Firefox does not. This would be a simple, super nice upgrade.

[hero1318]

@Kevingee  - FWIW, I just updated to FF 153.0 this morning and my layout.css.dev/PixelsPerPx setting stayed the way I had it set. I actually upped a bit more today, but this setting change still only works for toolbars across the top, not for sidebars.

So at least they've done part of what's needed here by not overwriting that layout setting during updates.  They just need to apply it to sidebars, or create a new layout setting specifically for sidebar fonts.