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!
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 email@example.com 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.)