Anyone wanting a replacement... (2) Rated 5 out of 5 stars

Yes, Nautipolis or Nuvola + CuteButtons Crystal SVG + NewScrollbars aka Noia Scroll. + Theme Font & Size Changer (font size = 19) : and Thunderbird remains my favorite Email client.

Thank you David Vincent Rated 5 out of 5 stars

Yes, it's a pity. It doesn't work anymore for Thunderbird 45.0.
David V. has already explained why...
Now, I use Nautipolis for Thunderbird (not so bad, alternative...)