Would you mind terribly telling us what exactly you are seeing that you don't like, where you're seeing it, what version of Thunderbird you're running, and what your OS is? We may be able to help you fix this if we know the details of what you think is broken. For now, please go to Settings > General > Language & Appearance > Fonts and Colors, use the Default Font drop-down to choose a font you like, then in the Size drop-down, choose 17, and then in the Advanced button menu, choose Latin and set the higher number to 17 and the lower number to 14 and then hit OK. Close and restart Thunderbird. Try it. I think you'll like it.