This must mean that EPM developers have dropped remote XUL in favor of some other web interface development technology. I tested Firefox 31.7.0 ESR and indeed the browser works just fine without installing the Remote XUL Manager extension:
|Look mum, no extensions!|
I found EPM Workspace working without a hitch.
The good thing about this change is that Firefox is now easier to deploy because you don't need to worry about distributing and configuring another browser add-on.