basings
commented
2 years ago MrOtherGuy has written a first draft for a working solution here
/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/urlbar_container_color_border.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */
/* Draws a border around urlbar using the color of the current container. */
#urlbar:not([open]) > #urlbar-input-container{
border-width: 0 !important;
padding: 2px !important;
}
#userContext-icons::before{
box-sizing: border-box;
display: block;
position: absolute;
content: "";
left: 0;
top: 0;
width: 100%;
height: 100%;
outline: 1px solid var(--identity-icon-color);
border-radius: var(--toolbarbutton-border-radius);
outline-offset: -1px;
pointer-events: none;
}
#urlbar[focused] #userContext-icons::before{
outline-width: 2px;
outline-offset: -2px
}
/* These might potentially cause some issues, but are needed to make the border extend the border to the full popup when urlabar popup is open */
#urlbar[open] > #urlbar-input-container{ position: static !important; }
#urlbar[open] > #urlbar-background{ z-index: -1 !important; }
/* OPTIONAL FEATURES */
/* Hide container label in urlbar */
/* #userContext-label{ display: none } */
/* Hide container icon in urlbar */
/* #userContext-indicator{ display: none } */
/* Hide the container line above tabs */
/* .tab-context-line{ display: none } */
This might be heavily based on preference, but if the whole address bar is colorized it's unambiguous that the website is open in a specific container.