I'm the dev who is responsible for the Windows implementation. It has been quite the... gnarly experience to say the least. We were shooting for 52 for enabling a11y+e10s, but it is increasingly looking like we're going to need to slip to 53.
Note that the approach we are taking on Windows is different from the way that Chrome works. They cache the entire a11y tree in the parent process so that the DLLs injected by assistive technologies may quickly enumerate the a11y tree via virtual function calls.
In 2017 we want to start moving away from injected DLLs in Firefox. As part of that initiative I decided not to take the caching route in a11y as that would require continuing to allow injected DLLs for a11y purposes. Instead I am implementing a COM handler that should allow us to significantly reduce the number of round trips required to enumerate the tree. I am hoping that this will offer AT vendors a migration path that eventually moves them out of our process.
Feel free to join us in the #accessibility channel on Mozilla's IRC server to chat further. I am aklotz on that server.
