For the few good reasons Google had for restricting addon manifests: performance and security. Declarative domain lists are easier to cache and lead to fewer (unnecessary) addon activations. Fewer permissions means the impact of a malware-infected version hitting the addon store in the future is a lot lower. uBlock's rule engine is incredibly powerful, to the point where a custom ruleset can inject code into any website. That applies to custom rulesets, but also to the built-in ones that may or may not get their accounts/hosting hacked, or bought out in the future.

Not that I would use the lite version myself, or that I agree with Google's choice, of course; they killed ad blocker APIs without providing an alternative API, after all. With the code already out there anyway, for the people stuck in their ways still using Google Chrome, they may as well make this version available for Firefox.