Thunderbird definitely does have autosync nowadays. No tray icon, true, but it can send native notifications which isn’t much worse.

It’s not much of a stupid question even given that - for a refractive medium, speed of light can change with its movement. Though for air it’ll be extremely hard to directly notice; it has n≈1.0003 so speed of light in air is already 0.9997c, and increasing it to, say, 0.9998c would require moving the air at 0.166c.