For on the fly css editing, you can use cssedit2. It's alot better than firebug's css editing, although firebug still has a bunch of useful features (like showing the box model attributes of a DOM item). And CSSEdit2 use's safari as it's preview browser. I tend to use firebug way more than safari's debugging tools.

The main reason why i stick with firefox is dTa & AdBlock plus. Safari has some adblock extensions, but their awkward to install and just dont work as well as AdBlock plus. Linkification, tamper data (lets you edit your http requests), the bugmenot extension, PicLens and what not also rock too.