Textmarker! is a text higlighter and text copying tool for Firefox. It allows you to highlight text selections via keyboard and context menu and then highlighted text to be copied. 3 highlighter colors can be configured.
Here is more about this add-on from the developers:
Allows you to highlight text selections via keyboard and context menu. Also allows for the highlighted text to be copied individually or all. 3 highlighter colors are available that can be customized.
Want to learn more? Check it out at the Firefox Add-on website.