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.