I'm going to have to write a client/server URL opening system that runs over SSH, aren't I. Run the server locally, it SSHs to your remote session and starts the client, which listens for requests to open a URL, passes them to the server, and the server spawns appropriate local Firefox or whatever processes.
I can do this with X, but there are issues, including that Firefox increasingly wants to use D-Bus for remotely opening URLs, not X properties.