[Newbie] Installing from DMG Files. Am I doing it wrong?

I've had a quick search but couldn't find an answer to this so was hoping someone could help.

When I open a .dmg file (say to install Firefox), the image is mounted and the window appears asking me to drag the Firefox icon to the Applications icon.

When I do this the icon appears in my applications folder on the dock but it's greyed out with a big crossed-circle over the icon. If I drag the icon to my desktop and then back again into the applications folder then the icon displays fine.

This has happened for everything I've tried to install so far.

Am I doing something wrong?

Sorry for the newbie question :-(

Cheers