The BrowserAPI is certainly not complete, but it's something. Some stuff might be harder than others. Intercepting networking is not very easy in Firefox – you can spy on networking, but the code isn't setup with hooks to change it up. And I don't know if something like localStorage would be exposed via the BrowserAPI – it's more low-level than that sort of thing. But if you are thinking about moving that kind of state around there's another Mozilla project called PiCL: https://wiki.mozilla.org/Identity/PiCL – it's working more in the guts of Firefox, but like much of Firefox it's in Javascript (compared to Chrome which is more biased towards C++). But it's not nearly as friendly an environment as content Javascript :( Anyway, that's more where localStorage manipulation and transportation is going to be happening in Firefox.