The problem with their tabs is that they put different things on the same sized window. I like my overall Tbird to be almost full screened. But I like messages relatively narrow, because I (and people) generally read less efficiently if the lines are really long; it's harder to track back to the beginning of the next line accurately.

I like tabs in urxvt, vim, Firefox, etc. But they don't work well for me in Tbird.