HighlightHunter от Roman Slezak

Word Highlighter is a lightweight and easy-to-use Firefox extension that lets you highlight any word on a webpage by simply double-clicking it.

Features:
– Double-click a word to highlight all its occurrences.
– Triple-click the same word to change its highlight color.
– Right-click anywhere to clear all highlights.
– Toggle the highlighter on/off via the toolbar icon.

Note:
After installing, make sure to pin the extension icon to your toolbar (right-click the icon and select "Pin to Toolbar") so you can easily toggle the highlighter on and off.

This tool is perfect for quick reading, studying, language learning, or simply keeping track of important terms while browsing.

Works on all websites and remembers highlight settings per tab.