Firefox doesn't come with a separator item in the customize palette.

Do you have the Classic Theme Restorer extension?

Yes, I do have Classic Theme Restorer installed (I love it!), but missed these customization nuances. After reading more threads, searching, and reviewing prefs.js, I saw the ctraddon_puib_separator.

A while back, I had problems with vertical separators, specifically with AIOS, and had also looked for horizontal separators. The solution at that time was customizableui-special-separator{d}, where d was a unique digit.

I always found the customization menu to be difficult to use, especially if a buttons addon was installed, mainly because it was unordered. I recently installed Sort and Search Customization Dialog and it has made a world of difference.

I just started changing my FF ways and using the new menu. I wanted to divide up the menu into sections or activities. On top I'd have a row of user tools like history, bookmarks, then a divider and have a row of customization tools, another divider and development tools. The divider which provides the visual separation and grouping is really nice. I hope FF adopts them. I'll take my questions and feedback to the Classic Theme Restorer Forum thread shown on the addon page:

https://addons.mozilla.org/en-US/firefox/addon/classicthemerestorer

Thank you.