Firefox Sync does work!
RPC is not currently supported but I agree it would be pretty interesting, tweety[0] was recently shared with me and that looks like it'd be quite nice, although I haven't tried it yet.
So far the only divergence from Firefox that could impact security is evaluating the config file, so I've described how that is sandboxed in the security[1] docs but I'm not super happy with the contents of that docs page; anything else you'd like to see mentioned?