Performance does suffer, because the operating system doesn't know that all that memory being used by firefox is just cache. And firefox doesn't know when the operating system would like to use memory for something other than cache. So when you start a second large program, it's thrashing time.
VMs like vmware have the exact same problem, where the host might want memory used by the guest, and you end up with weird scenarios where the guest's swap is in the host's disk cache, but the guest's memory is in the host's swap. One of the things guest tools are supposed to do is communicate with the host regarding memory pressure. Firefox lacks this feedback mechanism.
VMs like vmware have the exact same problem, where the host might want memory used by the guest, and you end up with weird scenarios where the guest's swap is in the host's disk cache, but the guest's memory is in the host's swap. One of the things guest tools are supposed to do is communicate with the host regarding memory pressure. Firefox lacks this feedback mechanism.