Ok, to start out, I’ve never used Firefox until recently–Safari rocks my Mac– but I decided to try out the newest version, and I actually like it– not as much as Safari make no mistake However, I don’t like it as much as Safari, but there are some steps to help with that:

• GrApple Themes – Themes that duplicate Safari’s interface, which are just awesome (Apple Fan, in case you didn’t notice)
• The aptly named Firefox-Mac-PDF which as the name suggests resolves Mozilla’s lack of a built-in PDF viewer in the browser, a feature included in Safari.
• Foxytunes - Eli already mentioned it here but its universal music control from your browser and it looks particularly nice in the themes Revolution or Revolution lite which match the whole OSX look above.
• Googlepedia– Aidan highly recommends this addition. Removes the Google ads on the right side of the search page, and replaces it with the relative Wikipedia entry.