Highlighting can be done with simple html, as described in the reviews section here. Any color can be applied with the span tag, or yellow with mark tags (see picture).

Thanks...but sadly I'm not a geek. Would prefer a button on the menu bar that does it automatically, like most other email/word processing programs do.

Yes, it would be nice to have the convenience of the add-on, but this is hardly a complex coding task. Select the text you wish to highlight, Insert/HTML..., add the mark tag before and after the text, OK.

Here is your source for info on this add on. https://addons.thunderbird.net/en-US/thunderbird/addon/highlighter-222207/ Add on authors are responsible for updates to their software.