ColorTabs For Firefox by Tetryl

A Firefox extension that lets you color-code any tab independently using a color picker or a set of favorites. Colors are tied to the individual tab, not the domain or bookmark.
Features

Right-click any tab → Color this tab — pick from your favorites or choose a custom color from a full color wheel
Favicon overlay — a colored dot is composited onto the original favicon so you see both the site icon and the color indicator at the same time
In-page top bar — a 3 px colored bar is pinned to the top edge of the page for an immediate visual cue whenever you are on a colored tab
Toolbar badge — the ColorTabs button in the toolbar shows a colored badge for the active tab, including on privileged pages like about:newtab where content scripts cannot run
Persistent colors — colors survive tab restores after a browser restart via the Sessions API
Favorites management — click the toolbar button to add, rename, recolor, or delete favorites; changes rebuild the context menu instantly
Custom color wheel — "Choose custom color…" opens a small popup with an <input type="color"> wheel, hex input, and a recently-used colors grid