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)

Context Menus (with lots of redundant and unused options removed):
Address bar dropdown
Page context menu
Image context menu
Link context menu

If anyone is interested in trying it out themselves, here is the UserChrome.css (which needs to go in the /chrome directory of your Firefox profile).

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.