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.
I had severe memory problems. Then on upgrade to, I think it was, FF11 firefox became so crash-prone that it was crashing on about every fifth page. Even "safe mode" wouldn't help.
Then I created a new profile and it was amazing. FF has been running without crashes since (not one) and memory usage seems well balanced.
I can't be the only person who 1) totally doesn't understand why these exist (I strongly suspect they're a hangover from DOS-based MS Windows without proper user accounts), and 2) how to manage them (there are no profile management tools within Firefox that I'm aware).
I use Kubuntu with a single user shared with my wife. Profiles mean we can easily have different FF set ups. I could do this through separate users but dont want extra administrative overhead.
I've heard of others using this as a way to keep a slim browser but still have a swiss-army webdev browser for working with.
It's not the profile system that was broken it was some user settings that were dragged in through the upgrades from version 3 (when I last replaced the profile).
Even less to just click an alternate icon in the app menu.
You wouldn't need to do user switching per se anyway. You could just "su" with the other users ID. It's the things like fixing a printer adjustment twice. Getting a password so I can fix something or share a file. Fine in a supported env. but for home I'm happy sharing the same user (but not with the kids!).
Which, given the lack of CLI invocation of the browser by virtually anyone (I freaking live in bash and I still launch my browser via a window manager icon) makes me think it's highly underutilized.
Mostly, Firefox profiles mean I can't reliably generically script actions on my Firefox configuration (given there's some arbitrary random string involved).
And if you use Firebug, you are screwed as well, only quicker.
I never really liked Chrome, but it's so fast and snappy, and Firefox has still not catch up in that area.