Source: http://lifehacker.com/5587822/find-all-enhances-text-search-in-firefox
Find All replaces the built-in search with a more powerful search tool. Whenever you search for a word or phrase a window will open at the bottom of the page with all the search results for the text. You can easily jump from one section of the text to another using the window of search results to navigate or you can simply read down through the text with all instances highlighted.
You can customize the search results including the size of the font, the color of the highlighting, and the layout of the toolbar. Find All is a free extension and works wherever Firefox does.