A super extension that adds a lot of niceties and exposes many of GitHub’s hidden features. A must-have for serious development work. It’s always been well-maintained and kept up-to-date with GitHub’s continual UI changes.

Sublime addon

I don't sync my extensions, as I use different ones for different profiles, but on nearly every profile, this is without a doubt one of the very first extensions I install on a fresh installation. It's also THE best developer add-on available for Firefox, and I strongly recommend anyone who uses GitHub regularly to install it. Feature-rich, stable, updated all the time...what's not to love?