There have been several highlighter extensions I have seen over the years for Firefox, however this one might be my favorite.  The ultra light-weight Highlights add-on for Firefox simply highlights selected text on web pages with a right-click of the mouse.

To highlight text on a web page, select the text, right-click (‘Ctrl+Click’, on Mac OS X) and select ‘Highlight’ menu from the popup.  The default yellow highlight color can easily be changed, by clicking the “Highlights” icon on the status bar, and choosing the color picker.  To remove the highlights, simply refresh the page.

No matter if you are doing research or you just like to highlight interesting points while you read, the Highlights extension is an excellent way to mark your spot on the Web.