The work is mostly related to browser vs AT navigation paradigm on Windows, however I imagine at-spi2 support within Firefox might also be affected mainly if they change some shared API in Firefox.
Nor at-spi nor orca currently uses techniques they are about changing so if a possible compatibility will arise it won't be planned.
I would advise to test with nightly version of Firefox so we can quickly discover issues as they arise during this period when Firefox accessibility is going to be in partial rewrite. Perhaps when we can gently report issues, we might get lucky and more of the a11y bugs will get solved at Firefox side of things. I guess linux a11y has not been touched for ages within Mozilla code base.
