It looks like the XUL Elements tabbox and tabpanels are currently broken for Linux users:
The tabs do not have any border or other visuals, the panel also has no border. I have heard this from multiple Linux users and it is not a theme issue as none of them had any special theme installed.
The last user reporting this to me is running Ubuntu 18.04 LTS with Gnome3 and Thunderbird 60.
Is this a bug in Thunderbird? Can we do something About it, or is it a fault in Linux?
Thanks for your help,