At around 06:49 CEST on the morning of August 27 2014, Google deployed an HTTP/2 draft-14 implementation on their front-end servers that handle logins to Google accounts (and possibly others). Those at least take care of all the various login stuff you do with Google, G+, gmail, etc.
The little problem with that was just that their implementation of HTTP2 is in disagreement with all existing client implementations of that same protocol at that draft level. Someone immediately noticed this problem and filed a bug against Firefox.
The Firefox Nightly and beta versions have HTTP2 enabled by default and so users quickly started to notice this and a range of duplicate bug reports have been filed. And keeps being filed as more users run into this problem. As far as I know, Chrome does not have this enabled by default so much fewer Chrome users get this ugly surprise.
The Google implementation has a broken cookie handling (remnants from the draft-13 it looks like by how they do it). As I write this, we're on the 7th day with this brokenness. We advice bleeding-edge users of Firefox to switch off HTTP/2 support in the mean time until Google wakes up and acts.
You can actually switch http2 support back on once you've logged in and it then continues to work fine. Below you can see what a lovely (wildly misleading) error message you get if you try http2 against Google right now with Firefox:
This post is being debated on hacker news.
Updated: 20:14 CEST: There's a fix coming, that supposedly will fix this problem on Thursday September 4th.
Update 2: In the morning of September 4th (my time), Google has reverted their servers to instead negotiate SPDY 3.1 and Firefox is fine with this.