I think it’s great too. It does an especially good job balancing its cleverness in a way that works well in each context it was designed for.
Somewhat aside: as a long time Mac user, it bums me out how the current macOS/iOS design language distracts from the logo as an icon. The Windows variation is much better, and would’ve fit right in on a Mac just a few years ago.
In all fairness, Thunderbird could have opted to use the windows/Linux logo for macOS and it would look amazing in the dock.
The silly white background squircle trend was set by the chromium/electron apps and it’s the opposite of the Human Interface Guidelines which recommend a distinctive and rich app icon.
I hate to be generous to Electron apps, but this wasn’t a trend set by them. The squircle thing happened directly in lockstep with macOS setting the expectation. Really good icons, even for Electron apps (VSCode is a prominent example), became indistinguishable practically overnight when the macOS design language was announced.
Unfortunately, Cupertino endorsed the squircle and encourages to “embrace simplicity”, so indistinct icons are recommended in the HIG now
“In macOS, app icons share a common set of visual attributes, including a rounded-rectangle shape, front-facing perspective, level position, and uniform drop shadow.“ [1]
Apple's new HIG is an exercise in marketing overriding usability unfortunately. I find it exceedingly hard to distinguish the squared icons on my Mac nowadays, and I also think they look worse than the more free-form ones from before.
At least it is still possible to use custom icons in Mac OS. But don't tell Apple!
Somewhat aside: as a long time Mac user, it bums me out how the current macOS/iOS design language distracts from the logo as an icon. The Windows variation is much better, and would’ve fit right in on a Mac just a few years ago.