I hope you're right about Firefox. I think their hand is forced in terms of supporting any API additions that Google makes because of Chrome's dominance among extension developers, but they can choose to maintain support for the manifest v2 features that are getting removed.
I would love to hear more about your personal project ideas that involve remote-browser. My contact info is in my profile, feel free to reach out.
however I see no reason why firefox should follow chrome on this? Also my natural paranoia in regards to the motivations of Google make me think maybe executeScript is too powerful and allows the extension makers to do things that are not malignant, but just against the corporate interests of Google.