Same with the Firefox Developer Edition logo. Damn near the same logo. https://www.mozilla.org/en-US/firefox/developer/

Given that the stable, dev, beta, and nightly logos used to basically just be palette swaps, they're supposed to be. PLus, "dev" firefox is literally the same version of Firefox as stable, but with a few about:config flags set differently by default. It's not even a different release version =)

I think GP means the new Thunderbird logo looks way too similar to the Firefox dev logo.