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 "light Noia ui" should be enabled, the corresponding line has to look like this:
@import "./css/noiaui_light.css";
If "light Noia ui" should be disabled, the corresponding line has to look like this:
/* @import "./css/noiaui_light.css"; */
Note
Code between /* and */ won't be used by Firefox unless there are other /* or */ inbetween.
Firefox 57-60
Enable once:
1. Tools > WebDeveloper > Toggle Tools > Toolbox Options > Enable browser chrome and add-on debugging toolboxes
2. Tools > WebDeveloper > Toggle Tools > Toolbox Options > Enable remote debugging
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 'disable popup auto hide' button (= button with four squares) on developer toolbars end.
Firefox 61+
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
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/noiaui_light.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.
[Buttons]
Noia button icons for toolbars (default)
Firefox 1 button icons for toolbars
Firefox 2 button icons for toolbars
Firefox 3 button icons for toolbars
Firefox 4 button icons for toolbars
shrink button icons on hover (default)
autosize for button icons on nav-bar (default) [compact=24/normal=30/touch=36]
18x18px (tiny) size for button icons on nav-bar
24x24px (small) size for button icons on nav-bar
30x30px (normal) size for button icons on nav-bar
36x36px (large) size for button icons on nav-bar
[Location Bar]
[Themes for toolbars and tabs known from 'Noia4' and 'NoiaFox' themes *]
[Additional theme settings]