I get it that Thunderbird looks unhip but I feel like the heavily padded lines waste too much space. The gmail "compact" setting is a much better compromise between space and legibility.

But unless they move Thunderbird to WebExtensions (which would be a total productivity disaster) it can always be modded.

Although an UI update is always welcome there are more important issues to address in Thunderbird, like better calendar synchronisation and an overhaul to its search capabilities.