I own the Pipelining Firefox extension, which I’m getting warnings about incompatibilities. However, it is kind of hard to update it when I can’t set about:config values. Any ideas?
Like Noit said, WebExtensions won’t implement direct access to preferences, but it’s possible to request APIs for specific features. In your case it would be some sort of HTTP Pipelining API. It seems unlikely that such a low-level API would be approved, since it’s known to cause some problems to users, but you can try and request it here.