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.
I still don't understand why there is almost no push towards restartless extensions on the AMO site apart from one "restartless" badge for addons. The front page[0] has literally no mention of them. For a while, they didn't have too many restartless add-ons. Recently, they've been getting more popular; Adblock Plus added restartless support a while back, after much work, and there are quite a few others [1]. It would be a decent time to start featuring these much more prominently, if not exclusively.
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.