If you are creating or maintaining Netscape style browser plugins on the Mac, you must get the nightly build of the open source version of Safari from WebKit.org. As someone who occasionally has been reduced to doing printfs to the system log in order to debug plugins remotely on a 10.3 box, it is luxurious having a debug build of the host application. Just follow the instructions for compiling a debug version and debugging it from XCode; if you have everything setup correctly you can debug the application and the plugin at the same time. Total setup time of less than an hour. It's fantastic.
I've tried to compile Firefox from source (using Visual Studio under Windows) and it was nearly impossible to get a debug build up and running. Actually, I don't know if it is nearly impossible or simply impossible since I gave up after a couple of days. Creating a Safari debug environment is a snap by comparison.
And the WebKit team is unbelievably responsive to bugs you find in WebKit itself. I've reported 2 problems over the last week and both were solved by the next day.