Firefox

1. If you have compact icons enabled, switch to normal
2. In about:config (enter into the address bar) turn it on:
   toolkit.legacyUserProfileCustomizations.stylesheets true
sidebar.revamp true
3. The native auto-hiding of the sidebar should be disabled (In the sidebar settings)
4. Create a "chrome" folder in 'Profile Folder' (Menu > Help > Troubleshooting Information > Profile Folder)
5. Copy there userChrome.css
6. Restart Firefox

Use padding around the edges of the address bar to scale and move the window.
Drag Bookmarks Toolbar Items to the left of the address bar.

Sidebery

1. Install Sidebery
2. Enable [Sidebery] prefix in the Sidebery settings. The enabled Sidebery (by icon or hotkey) replaces the native side tabs.
3. Copy code from Sidebery.css to Sidebery / Settings / Styles editor / Sidebar

4. Copy code to Styles editor / Group page

#root[data-frame-color-scheme=dark] {--s-toolbar-bg: hsl(252, 4%, 9%) !important}

html {background-color: var(--s-toolbar-bg) !important}

.title {opacity: .7}

YouTube

1. Install Stylus
2. Open YouTube
3. Stylus: New Style > Import > Copy code from YouTube.css > Replace style > Save

To open the header, move the cursor to the top edge, but not all the way.

about:config

Thin scrollbars widget.non-native-theme.scrollbar.style 3
Open a bookmark in a new tab browser.tabs.loadBookmarksInTabs true
Open tab after current browser.tabs.insertAfterCurrent true
Smoothness of scrolling general.smoothScroll.mouseWheel.durationMaxMS 380
Disable add-on recommendations extensions.htmlaboutaddons.recommendations.enabled false
Restore pinned tabs on demand browser.sessionstore.restore_pinned_tabs_on_demand true
OS text scaling settings should only affect text scaling (prevents blurring of icons) browser.display.os-zoom-behavior 2
Trim https browser.urlbar.trimHttps true
Decode URLs on copy browser.urlbar.decodeURLsOnCopy true