If you happen to use a (Mozilla) Firefox Nightly build and want to access GMail or any Google service, you can’t. Not without a configuration tweak. Nightly since v34 has broken all Google Accounts 🙂 And no its not a cookie handling issue even if that’s what the post-login error screen implies.
All you have to do is open a new browser tab, and type “about:config” (without the double quotes of course) into the address bar to access the browser settings.
In the Search field (displayed at top of screen just below the address bar), type “network.http.spdy.enabled.http2draft” (again not with the quotes) and switch it from True (the default) to False.
Then just to be on the safe side go purge all Google related cookies.
Now when you access a Google Account (Google Plus, GMail, YouTube, etc) it will open with no fuss
Of course if you type in the string mentioned above and don’t find a match your Nightly version is ancient and the problem lies elsewhere.