v60 Photon style is terrible

[Shepsl]

I frequently keep multiple emails open, so having tabs that are clearly discernible was a real plus. The so-called Photon design with rectangular tabs (from Firefox) is terrible. The tabs visually run into one another and the menu bar above and all in all are harder to read. Bad, bad design decision.

[tanstaafl]

The CustomizeMyBird add-on lets you specify curved tabs, among other things.

Thunderbird uses the Mozilla toolkit, whose design is driven by the needs of Firefox. This is also the reason many add-ons broke in version 60.