I suggest removing the card borders that make it difficult to read the content and identify unread emails. This will give the text more room and reduce eye strain.
While waiting for an official solution, I created a userChrome.css, although it is still far from what can be done (such as improving the identification of unread emails or even could maintain the the blue left rounded border to selected email, currently implemented in TB 145).
Overall, it would be better if the user could have more control over the overall appearance, being able to customize these and other visual aspects of TB (maybe similar to Firefox Color).