Browser Upgrades - MAC

As with most applications for the Mac, once you open the dmg (image) file, it's a matter of dragging the application's icon into your system's Applications folder. 

For Firefox, download the latest Firefox version for OS X at www.mozilla.com. Once downloaded, a "Firefox 2.0dmg"  file appears on your desktop. Open the .dmg file and drag the Firefox application inside it into your Applications folder.

If you already have an earlier version of Firefox in your Applications folder, OS X will prompt you saying an older version of this application already exists and do you want to replace it.  Say yes and it installs.  Double-click the replaced application and the updated version of Firefox should open.

More installation/uninstalling directions can be found here:   
http://www.mozilla.com/en-US/firefox/2.0.0.1/releasenotes/

Safari is built-in as part of OS X.  Periodically there may be small updates to Safari which you can check by clicking the blue apple on the upper left-hand corner of your screen and selecting "Software Update" from the drop-down list.  You can also check on Apple's website in the OS X downloads section.