Highlight Opened Tabs is a lightweight Firefox extension that helps you see which pages are already open. It highlights existing tabs when you hover a link on any page and highlights duplicates of the currently active tab, so you can avoid opening the same page twice. Highlight opened tabs on hover Hover any link on a webpage – tabs with the same URL are highlighted in your tab strip. The hover delay is configurable, and highlights can be cleared automatically when you leave the window/top panel. Highlight duplicates of the active tab When you switch or load a tab, all other tabs with the same normalized URL are highlighted. An optional badge counter shows how many duplicates are open. Technical details - Built on Manifest V3 - Uses a lightweight content script and event‑based background logic - No data collection, tracking, or analytics Localization The extension is fully translated into 19 languages, including: English, Spanish, German, French, Russian, Belarusian, Chinese (Simplified and Traditional), Japanese, Portuguese, Italian, Korean, Polish, Dutch, Turkish, Ukrainian, Indonesian, Arabic, and Vietnamese.