There is a way using html. Click on 'Write' compose text. eg: Gone fishing Highlight a word eg: fishing 'Insert' > 'HTML' Enter the html code as shown in image below. click on 'Insert' button.

Thank you. It is a bit archaic however it will work. I was hoping for an easier way, LOL. I can also now see the HTML within the page.

I am using Highlighter version 0.6.5 with Thunderbird 52.4 and it works just fine. https://addons.mozilla.org/en-US/thunderbird/addon/highlighter-222207/?src=ss

I'm running TB 52.7.0 and Highlighter version 0.6.5 works for me as well.

https://addons.mozilla.org/en-US/thunderbird/addon/highlighter-222207/?src=ss