Rated 5 out of 5 stars
I used Thunderbird but hated the very small size of text. After fussing around with adding a userChrome.css to Thunderbird I still wasn't satisfied.
Using Theme Font & Size Changer I was immediately able to change the size of the font so that I can actually read my email without squinting and getting a headache. The Add-on is so simple to use - and makes an enormous difference. Thank you!