Windows/Mac/Unix (Firefox): The HighlightAll Firefox extension highlights any text on a web page matching the text you’ve selected with your mouse.
Like most good Firefox extensions, Highlight all does one simple thing well. This performs the same function as the “Highlight all” button when you’re searching a page with Ctrl/Cmd-F, without the need for typing or watching your screen jump around to matches as you search. After all, you’ve already found the text you want highlighted, so just select it and you can see all other instances. You can toggle HighlightAll with a configurable keyboard shortcut (F8 by default), so matches to your highlighted text doesn’t always need to show up in yellow. This seems like a great research tool for quickly digesting web pages by keyword.