One of the most common Mac newbie mistakes is running an application from within a disk image. For some reason, this is most often the case with Firefox.
You can think of a disk image like a box used to physically deliver your application; you need to receive the box (download), open the box (double-click disk image), and move the contents out of the box and into your home (drag application icon into your applications folder).
Typically, when you download an application from the internet, it comes “wrapped” in a disk image. If you’re coming from PC land, a disk image is similar to a Zip file, in that both disk images and zip files contain several files within them.
Let’s use Firefox as an example of the correct way to install an application from a disk image. Go to the Firefox download page and click “Download Firefox” – you will see that the file you are downloading has a “.dmg” extension. This is the “disk image” extension.
Once downloaded, the disk image will probably try to open automatically – you should see the End-User Software License Agreement:
If you don’t see the above agreement, you’ll need to go to your download’s folder (if you’re running leopard, the default location for Safari is the Downloads folder within your home folder) and double click “Firefox 2.0.0.12.dmg” (or whatever the current version is). If you can’t find it, try searching “Firefox” in Spotlight (apple+space bar).
After you press “Accept” on the End-User Software License Agreement, you should see this:
What this screen is telling you is, “drag Firefox.app to your Applications folder.” This is where most people go wrong – they just double click Firefox.app and start using it. Although this may seem to work, certain features may not work correctly, some settings may not be saved, and the program may crash.
Anytime you download a disk image which contains an application or folder (i.e. Firefox, Toast, Office 2004), you install the application by dragging the application’s icon or application’s folder (see Toast image below) to your Applications folder.
Firefox is an especially troublesome program for newbies to install because the Applications folder in the above image isn’t really your Applications folder (it’s just a background image). In this case, you must click the button at the top right of the window: 
. This button expands the current window to include the Applications folder on the left column. Now you can drag the Firefox.app icon to the applications folder:
Firefox has now been installed. Now you can eject the disk image (by clicking the eject symbol next to “Firefox” in the left column of Finder) and then delete the “Firefox 2.0.0.12.dmg” file. Run Firefox by clicking the Applications folder and double clicking Firefox. Once opened, you can keep Firefox in your dock by ctrl-clicking the Firefox icon in the dock and selecting “Keep in Dock.”
Some programs are more helpful than Firefox, like Toast, which actually includes your real Applications folder, making the drag-and-drop installation procedure even easier.
I should also note, some programs do include installer files (such as Microsoft Office 2008) which automate the installation process. In this case, instead of dragging the file to your Applications folder, you would open the disk image, or load the cd, and double-click the installer application (which is usually called something like, “double click to install office 2008”).
