I confirm, the bug is still actual for Ubuntu 18.04. Using Thunderbird 52.8.0.
In the same time it does not actual for Ubuntu-16, cause that version is using <Unity>. The Ubuntu science 17v start using <Gnome>.
So that, the problem, more probable, is connected with Thunderbird GUI framework which is relying to specific graphical engine functions.
As I see, Thunderbird-team should investigate the problem.
I confirm, the bug is still actual for Ubuntu 18.04. Using Thunderbird 52.8.0.
In the same time it does not actual for Ubuntu-16, cause that version is using <Unity>. The Ubuntu science 17v start using <Gnome>.
So that, the problem, more probable, is connected with Thunderbird GUI framework which is relying to specific graphical engine functions.
As I see, Thunderbird-team should investigate the problem.