Yahoo has released a plugin that allows end to end encryption for yahoo mail users. It’s seems to be part of the new “yahoo” redesign:

we’ve heard you loud and clear: We’re building the best products to ensure a more secure user experience and overall digital ecosystem.

It’s open sourced and they also setup a bug bounty program (from 50$ to 15,000$)

While at this stage we’re rolling out the source code for feedback from the wider security industry

More on their tumblr (this sounds weird).

Glancing over the code it looks like it’s cumbersome to use:

The extension requires a keyserver implementing this API to fetch keys for other users.