Rated 5 out of 5 stars

A very good attempt. Small issue: The highlighting height appears to be constant and is only correct for "normal" sized text. Perhaps add a button to increase or decrease the highlighting text height.

Thanks so much for creating this add-on.

Edit: Highlight height appears to be a Thunderbird HTML processing issue.

Changed my rating to 5 stars!

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

I just tried to mark text passages written with small fonts and big fonts - the highlighting seems to be okay.
Can you please send an e-mail to k-l-p@gmx.de with some examples/pictures?
(By the way, my Add-on only sets the background colour and has no influence on text height or highlighting height.)