rolf wroteI use a combination of Chrome and Firefox. I have found Firefox to be more compliant and predictable then Chrome in some cases. Plus I like the extensions. But Chrome is good to open tens of tabs - it handles it real well. Safari is pretty much Chrome with a different "body" (if we use the car analogy), and it seems to be slower and heavier at start-up. I'm not very much at ease with Opera's interface, plus it seemed to have some stability and memory issues under heavy use. I don't know about the newer versions, though. IE has been doing lots of progress but still somehow feels like (and, I guess, is) low-quality software and outdated technology; and the countless "security features" are a huge turn off.
What I found is that Firefox 5 is the only Web Browser that runs in 64-bit mode in Mac OSX Snow Leopard.
Unlike Chrome, Opera does not go memory hungry. It consumes only about 180 MB with 6 Extensions, Opera Link and Turbo Mode Enabled. Each Extension and Plug-in in Chrome, is treated as a separate task in Mac OS. On my Mac, there are about 20 task, about 500 MB of Total Memory Consumption! This is crazy! If you are running it on a laptop, the battery life will be affected.