has been the default web browser on Mac OS X since 10.3 (Panther) and is the only browser for the iPhone and iPod Touch.
One of my major annoyances has been that new links would always often in a new window but that can easily be solved with the follow lines in Terminal.app:
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true
While there is no native version of Firebug to be used in Safari there is an alternative with the Safari web inspector. Once you've run the line below you can access the inspector by clicking on the Develop -> Show Web Inspector menu item.
defaults write com.apple.Safari WebKitDeveloperExtras -bool true
Make sure that is run with Safari closed and then next time you launch new links will be open in a new tab.