I work with a lot of Go (and so I review PRs written in Go), so my favorite extension is one that I wrote myself [0] which corrects GitHub's tab indentation to 2 spaces from the default 8 spaces. But then again, I only have a few extensions and I wrote the majority of them.
Knockout Context Debugger
uBlock Origin
User CSS
Simple REST Client
I prefer extensions that work without you creating an account on something. I am unlikely to use ones that require an account unless I am desperate to use it. I am also unlikely to use an extension when either a web site or desktop app will do.
uBlock Origin (Open-source efficient ad-blocker)
The Great Suspender (It allows me to open +200 tabs) (Open-source)
Session Buddy (Organizing tabs/sessions)
Sourcegraph CEO here! Thanks for mentioning our Chrome extension. It adds IDE-like code intelligence to GitHub.com and other code hosts (when viewing code files and diffs/pull requests): hovers, go-to-definition, and find-references.
The extension is awesome, it could just have a bit more polish in the firefox version (font and icon don't seem to load properly in the browser bar, at least in most recent version of the firefox dev edition). Thanks for your hard work on sourcegraph though!
