highlight it how? I drag with my mouse the same way I have highlighted text on windows since version 2.

Thank-you for the reply; however, Add-On Highlighter 0.6.5 does not work on Thunderbird version 52. Other users have made the same comment. I am having to use an archaic html approach to highlight: <font style="BACKGROUND-COLOR: yellow"> __</font>

This lack of highlighting ability on the most recent version of Thunderbird is surprising and disappointing as this is very basic email functionality.

Windows Live mail 2012 had all the required features.

The Highlighter add-on works here in TB 52.7 and TB 60:

https://support.mozilla.org/en-US/questions/1170298

Yes, it worked. Usually, when you download it appears on the extensions list and you click install. For some reason, it did not appear on the list of add-ons. In the download files of the laptop, I dragged the file into the Add-On list and re-started Thunderbird. The highlighting works now. Thank-you!