To clarify myself: I use SSH-forwarded remote X today, so today I can be on a remote system and feed URLs to my local Firefox via its remote control feature. It is just rather slow over my current DSL link, because it involves a bunch of round trips to manipulate X properties (and it requires a hacked Firefox that uses X-based remote control, which I have for now).
On Linux, Firefox's modern remote control normally uses D-Bus, making it intrinsically local-only AFAIK.