I use nightly as well as the developer edition (aurora) on a regular basis and also side-by-side. With the Aurora/Developer Edition Icon blue, those two browsers are hard to tell apart. I would prefer if the icon for the developer edition was in a completely different color (i.e., yellow, purple, red, green, .. :)

Seconding this, and wondering if there's an easy way of modifying it, like installing an addon that will change this icon, as I too use both aurora/nightly at once, and alt-tabbing fast winds up in me selecting the wrong one.

You can go back by getting an icon file and changing this yourself. We have no immediate plans to revisit the icon design. Personally, I'd want a martian-red nightly icon but I'm strange.