John Gruber regarding Safari:

The other big thing that’s missing (compared to both Chrome and Firefox) is a proper extension API. If only Apple had an imminent developer conference where they could unveil such a thing.

I continue to use Firefox for nearly all of my browsing and the addition of extensions wouldn’t compel me to switch. Currently, the only extension I have installed is Flashblock and I would actually prefer to use ClickToFlash. The biggest thing keeping me from using Safari is its lack of a “open new windows in a new tab instead” option.