Genuinely curious.
Now I wonder - maybe I got it all wrong? Does it actually allow to completely override built-in Firefox Sync (and either ditch Firefox Accounts requirement for Sync engine to start[1], or override those as well) and replace those parts with a custom implementation?
Say, I don't want any Mozilla nonsense, and want Firefox to sync with a simple WebDAV storage (rather than their proprietary protocol), and do so authenticated with a sane(r) X.509 client cert rather than an ugly mess of BrowserID, HAWK and OAuth2. Is that possible?
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[1] This part is probably easy through messing with browser's preferences (the ones in about:config), but I'm not sure there's a WebExtensions API for this. Have looked for it and haven't found anything.
If even those are removed or blocked, there won't ever be a way (ok, a way without explicit Mozilla's blessing) to implement custom password stores (think OS X Keychain, KDE KWallet or GNOME Keyring), or a custom sync engine, or tree-style tabs.
And with WebExtensions I can't find (maybe that's me, though!) an equivalent of preferences/service Add-on SDK API.