I've toggled between Safari and Firefox for the last few months. Safari has "seemed" a bit more responsive since the last Firefox upgrade - while Firefox has started to hang more under certain circumstances - which seems like a new phenomenon. I've heard from colleagues that Firefox has become a bit of a memory hog, but I'm not seeing that in the Activity Monitor. In fact, quite the opposite:
I'm also not noticing a huge difference between version 4 and version 5 in terms of performance. I've got Opera installed as well, and that actually uses the least amount of system resources. But all three seem to operate at about the same response time.
Just for fun, here are the comparisons between the three browsers at idle (which may tell me exactly nothing, but hey...)
I've always said that as soon as Safari got a Firefox-like search bar, I was on board and, low and behold, here we are.
I've been trying it out tonight in lieu of Firefox to see how I like it and I'm very impressed - the addition of extensions, search in the address bar ("awesomebar" from Firefox) and the "Reader" function have all made me pretty happy!
I don't do too much extension development but I was poking around with the extension builder and it seems very user friendly. I don't really have much experience building them, of course, but it seemed like it would be very easy to pick up and learn. Hopefully some of the Firefox extensions come over to Safari in the next few weeks.
In terms of speed, it's fairly fast - I've never been one to have an eye for comparisons of each browser but when I went to three of the sites I visit on a daily basis - cbc.ca, news.bbc.co.uk, and facebook.com ( and the-w.com!) - it seemed pretty fast to me.