userChrome.css

/* First, check these links:
& https://www.userchrome.org/
& https://www.askvg.com/tip-enable-colorized-titlebar-again-in-firefox-103-and-later-versions/

With this css, firefox will have colored title bar like google chrome.
If you don't like plain white, give it a try. 

In the article above, inactive window color is same with active window color, thus you can't distinguish the difference.
It makes harder to understand whether the window is selected or not.  This version fixes it.
*/


/* active window and title text color */
.titlebar-color:not(:-moz-window-inactive){
color: AccentColorText;
background-color: AccentColor;
}

/* inactive window and title text color */
.titlebar-color{
color: color-mix(in srgb, AccentColorText 75%, transparent);
background-color: color-mix(in srgb, AccentColor 75%, white);
}

/* not selected tabs transparent container */
.tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background{
background-color: color-mix(in srgb, currentColor 10%, transparent);
}