Writing a parallel GC surely is not "low-hanging fruit" by any reasonable measure. Java already has to deal with concurrency in their objects, and throughput is more of a concern for server-like workloads, so they are dealing with a different situation. In Firefox, most of our effort has focused on improving responsiveness and eliminating work (through things like compartmental GC). (Some GC sweeping is already done in parallel in Firefox.)

That said, I believe IE does some concurrent marking, so there is certainly room for improvement.