I mainly use my keyboard to navigate around in Firefox so decided to edit UserChrome.css to create a custom, ultra-minimalist "one line" UI for myself and also maximize my screen real-estate by removing the window Titlebar and Tab Bar (using Tree Tabs sidebar extension instead). I also dislike how cluttered the Firefox interface is with unneeded options scattered everywhere, and how much redundancy there is with many options showing up in multiple places for no good reason, so I removed most of that as well. Here is the results:
Main UI (Navigation and "Hamburger" toolbar buttons removed)
Tree Tabs sidebar & More Tools both open
"Find in page" moved to the top, with Menu bar also toggled on
New Tab Page (my Bookmark Toolbar auto-unhides itself only on this page)
My Home Page, set to the FF Library "popout" page (chrome://browser/content/places/places.xul)
And if enough people are interested in learning Firefox UserChrome.css customization using the Browser Toolbox with remote debugging, I can always write up a tutorial at some point. There are some decent resources already available over at userchrome.org and reddit.com/r/FirefoxCSS/ too.