End-to-end is a recently unveiled Google Chrome extension that adds OpenPGP capabilities to Gmail to allow for end-to-end encryption. I've given a quick cursory glance at the code (git clone-able from git clone https://code.google.com/p/end-to-end/), and it seems to very much be designed to be a Chrome-only add-on. As the web has many citizens using browsers other than Chrome, it might be a not evil thing to try to port this thing to Firefox, and see if it could be ported to the other browsers as well. Like I implied, however, this'll be an uphill slog. It's not a simple extension by any means, and it's Google Chrome dependencies seem, at least to my eyes, to be baked in pretty deeply. Anyhow, filing this to express support for such an effort. : Project home: https://code.google.com/p/end-to-end/, and original announcement: http://googleonlinesecurity.blogspot.ca/2014/06/making-end-to-end-encryption-easier-to.html
Silly me, provided a footnote without linking it to anything. Still, I think you get the idea.
For those interested in encrypting their webmail (not just GMail!) through Firefox, there is Mailvelope: https://www.mailvelope.com/