For a while on the Mac I kept finding myself always clicking on the URL/location field in Firefox and Safari. Then one day I noticed this, and wondered why I was doing that instead of using the keyboard command/shortcut to highlight the field and then typing. That answer was easy -- I didn't know what the keyboard command was. It's not the same as it was on Windows.
Turns out it's very simple:
[command][L] does the trick.
(That should be a lower-case letter 'L'; I just made it upper-case there to contrast it with other similar-looking characters.) The 'L' stands for "location".
This keyboard shortcut works on both Firefox and Safari on Mac OS X.