I'm always surprised with peoples experience with Firefox memory usage of recent, it has been excellent for me. I can sometimes get upto 200+ tabs, while only sitting on 1.5GB of used memory. This is in Linux, but most of the memory-handling code shouldn't be that different between OSs in terms of GC algorithms.

As for restartless extensions, I think mozilla should really push people to switch their extensions to use the api. The other advantage is version independence, which helps with some of users pains of firefox.