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
3. Create a "chrome" folder in 'Profile Folder' (Menu > Help > Troubleshooting Information > Profile Folder)
4. Copy there userChrome.css
5. Restart Firefox

Drag Bookmarks Toolbar Items to the left of the address bar.

To update without rebooting and live-editing userChrome.css

If you're writing the styles yourself, you can set up browser toolbox and edit the files directly in it without having to restart Firefox for changes to apply. (You still need to restart Firefox when creating userChrome.css). On top of being able >to live-edit userChrome.css, the browser toolbox is essential in figuring out how to add custom styling to various parts of Firefox.

You can find userChrome.css in the browser toolbox like this:

1. Open browser toolbox
2. Activate the Style Editor tab of the toolbox
3. Type userChrome.css in the 'Filter stylesheets' box on the left
4. Your userChrome.css appears on the right. Once you are done editing it, you can either close the toolbox which won't save your edits, or save edits by clicking the 'Save' link on the left.

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

To adjust Sidebery width: disable Sidebery > open Firefox's Side Tab Settings > adjust panel width > enable Sidebery.

Auto-hide Sidebery is enabled in the same way as Firefox side tabs.

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
Disable AI browser.ml.enable``false