> Is it impossible to support window or otherwise chrome-level event listeners and also e10s?
Yes, because those things are now running in different processes, which means it requires IPC, and allowing extensions to communicate over IPC with the chome throws all the benefits of e10s out the window.
It looks like it's been broken down into a number of separate issues since then, and there's some discovery yet to be done on how exactly the implementation will need to proceed - I feel like Firefox 58 is very optimistic, but I also feel like saying "this is never going to happen" is pretty premature at this point. At the very least, the Firefox devs don't seem to agree.
Yes, because those things are now running in different processes, which means it requires IPC, and allowing extensions to communicate over IPC with the chome throws all the benefits of e10s out the window.