Tab Colouriser av AuthenticRavenim

Right-click any tab → Colour Tab → pick one of nine colours, and the tab gets a coloured marker in the tab strip. Pick "Remove colour" to clear it. Colours last for the session. Under the hood it uses Firefox's Tab Groups feature: each coloured tab becomes its own single-tab group. Notes: colouring a pinned tab unpins it, and colouring a tab already inside a group recolours the whole group. Requires Firefox 142+.