Rated 5 out of 5 stars

Thanks! I have missed the highlighter in earlier versions of TB. Works as intended (yellow highlight of selected text). Toolbar button is on the right of the compose window; customize to place on the left if that is your preference. An option to change the colour would be nice.

This review is for a previous version of the add-on (1.0). 

I will work on introducing an options page to choose different colors.
Edit 04.12.2020: new version with options page and many colours available.