If I use Thunderbird with only the Retina screen of my MBP it's fine. However, if I connect my MBP to an external display with normal resolution, then Thunderbird won't scale properly according to Retina resolution if I move the app back onto the Retina screen, resulting in the fonts being extremely small. I'm not sure if this is an ubiquitous occurrence only happening in my case. If no, there might have already been a solution to it?
OK it seems to have been caused by a plugin called Theme Font & Size Changer. After I removed this plugin, Thunderbird renders properly.