• CustomCSSforFx - current release & changelog - Last version with 102 ESR support
• Custom JavaScript scripts for Firefox - NoiaButtons CSS tweaks

This project is dual-licensed under the GPLv3 and MPL 2.0, see the terms of the LICENSE files.

• Unlock custom CSS usage
• WebExtensions can not modify Firefox appearance properly
• Where to find Firefox profile folder? The correct location for user styles.
• How to use custom user styles?
• How to find item ids and attributes?
• How to modify custom user styles?
• Firefox Color (compatible with default color preset of CustomCSSforFx)

about:config > toolkit.legacyUserProfileCustomizations.stylesheets > true

The only way to modify ui is adding custom CSS code to userChrome.css and userContent.css files inside browsers profile folder.
Keep in mind CSS code can not create entirely new items, buttons or toolbars. It only can modify already present ui items.

1. Find your profile folder ('profile names are different for everyone').
about:support > Profile Folder > Open Folder
or about:profiles > Root Directory > Open Folder

2. User styles belong into \chrome\ folder. Create it, if there is none yet. It should look like this afterwards:
\ PROFILE FOLDER NAME \chrome\

3. Copy userChrome.css, userContent.css and \config\, \css\, \image\ folders into \chrome\ folder. It should look like this afterwards:
\chrome\config\
\chrome\css\
\chrome\image\
\chrome\userChrome.css
\chrome\userContent.css

(Optional) Profile folders location on drive:
Windows
C:\Users\ USERNAME \AppData\Roaming\Mozilla\Firefox\Profiles\ PROFILE FOLDER NAME \
Hidden files must be visible to see AppData folder. Alternatively open %APPDATA%\Mozilla\Firefox\Profiles\ from explorers location bar.
Linux
/home/ username /.mozilla/firefox/ profile folder name /
Hidden files must be visible to see .mozilla folder.
Mac OS X
~\Library\Mozilla\Firefox\Profiles\ PROFILE FOLDER NAME \ or
~\Library\Application Support\Mozilla\Firefox\Profiles\ PROFILE FOLDER NAME \
\Users\ USERNAME \Library\Application\Support\Firefox\Profiles\

The userChrome.css and userContent.css files works like an options\configurations file. All main "features" can be enabled and disabled there.
Edit userChrome.css and userContent.css with any text editor (Notepad++ recommended on Windows) and enable or disable any feature you like by modifying, removing or outcommenting available @import strings.
Restart Firefox after every modification for changes to take effect.

Example
If "classic button appearance for navigation toolbar buttons" should be enabled, the corresponding line has to look like this:
@import "./css/buttons/buttons_on_navbar_classic_appearance.css"; /**/

If "classic button appearance for navigation toolbar buttons" should be disabled, the corresponding line has to look like this:
/* @import "./css/buttons/buttons_on_navbar_classic_appearance.css"; /**/

Note
Code between /* and */ won't be used by Firefox unless there are other /* or */ inbetween.

Enable once:
1. Tools > WebDeveloper > Toggle Tools > 'Customize Tools and get help button' (= button with three dots) > Settings > Enable browser chrome and add-on debugging toolboxes
2. Tools > WebDeveloper > Toggle Tools > 'Customize Tools and get help button' (= button with three dots) > Settings > Enable remote debugging

or set these two in about:config to true

about:config > devtools.chrome.enabled > true
about:config > devtools.debugger.remote-enabled > true

Hit Ctrl+Alt+Shift+I or open 'Tools > WebDeveloper > Browser Toolbox'.

Inspect ui or web content.

Force popups to stay open for inspection: Click on 'Customize Tools and get help button' (= button with three dots) and select 'Disable popup auto-hide'.

Open CSS files with a text editor. Look through the code and change values the way you need.
Some files contain additional instructions about how to tweak the ui for individual cases.
Restart Firefox for changes to take effect.

Example
Open ./css/tabs/classic_squared_tabs.css file
Look for /* unloaded/pending tabs color *//*
Remove /* at lines end to make that part of the code active. Save the file and restart Firefox.

Example 2
Open userChrome.css file
Look for @import "./css/tabs/classic_squared_tabs.css"; /**/
Add /* at lines start to disable 'classic squared tabs' appearance.
The result should look like /* @import "./css/tabs/classic_squared_tabs.css"; /**/

Example 3
Open userChrome.css file
Look for /* @import "./css/locationbar/reader_alternative_icon.css"; /**/
Remove /* at lines start to enable the alternative reader icon appearance.
The result should look like @import "./css/locationbar/reader_alternative_icon.css"; /**/