The prostetnic highlighter, a Firefox add-on for highlighting text in web pages.
Get it at https://addons.mozilla.org/en-US/firefox/addon/prostetnic/
Select text in a web page. Multiple (non-contiguous) selections are OK, as are ones that straddle boundaries between paragraphs, tables, lists and any other HTML elements. Select colors for text and background from entire color gamut. Highlights will reappear if page is revisited, or highlighted text can be looked up.