I'm not sure what experience you're describing, for me the real crime trying to run a remote Firefox over a forwarded X session is that it figures out you're doing that and instead decides to run another window of your local Firefox instance. It's been like that a few years now. I can't imagine what they're doing under the hood, haven't looked into it, but it sure is irritating.
It's not detecting that you are trying to run remote. That's just the default behavior of the firefox command, to use the running instance of there is one. IIRC, there is an option you can pass on the command line to get around it.
