Installing software on the Mac is usually a no-brainer: You get a ZIP archive or disk image that gets uncompressed or mounted for you. What is left for you is to move whatever application to your Applications folder. Done.
In case of a disk image, developers often go so far to include a reference to your Applications folder on the disk image itself. So all you have to do is to drag an icon onto another one less than 200 pixels away. This works so nicely on the Mac (the advantages of a controlled environment) because in the majority of cases, it is at /Applications on your filesystem.
To make that even easier, developers usually put nice background images onto those disk images to illustrate (with an arrow for example), what the user has to do. Skype, among others, nail this experience. (I’m not talking about the ZIP-only approach here). They have, on their download page, a step-by-step guide accompanied by screenshots that guides the user through the process.
The Firefox team does the same and I guess with good intentions, but their attempt falls apart quickly.
The icon on the disk image that looks like a reference to your Applications folder is none. It is just a dead image. Boy is this misleading (considering the arrow) and annoying.
The disk image is configured to open in a sidebar-less mode. It hides the only actual reference to the Applications folder. But the instructions on the download page show it. I consider this confusing for Firefox’s target audience: The average, not computer-savvy, internet user.
I remember filing a bug for that ages ago, but I could not find it now. The accomplishments of the Firefox team are enormous and I do not want to discredit any of it, but please, do get this crucial bit (the first impression) right.