First, we need to identify where the problems - if any - lie. To do that, let's take a look at the common process of installing applications on the Mac:
- Download disk image
- Mount disk image
- Move application from disk image to ~/Applications
- Optionally add application to the dock
This is the four step process as described by Limi, but personally I would replace step four with "Unmount and discard disk image", but that's splitting hairs; they're both optional, but I personally believe removing the disk image is a more common operation than adding an application to the dock (which is something I usually do whenever I launch the application in question for the first time).
This process, while dead easy and familiar for experienced Mac users, is completely alien to new users. Just about any Mac application out there tries to solve this issue by visualising the process to new users. The best way I know how is to manipulate the Finder settings for the disk image; AdiumX does this very well:
Informal testing revealed two common errors being made by people new to the Mac, and the situation is only worse for Firefox because it's often the first application people go and download because they're familiar with it. One of the common errors is that people drag the application directly from the disk image to the dock. This creates a dock link directly into the disk image, leading to slow performance whenever the application is started when the disk image is not mounted.
The other issue is that people often open the application directly from within the disk image, which leads to users mounting the disk image every time they want to launch Firefox. This is of course a very round-about way of doing things.
Apple is aware of the problem, as a dialog pops up when you try to run an application from within the disk image, warning you about what you are about to do. However, this dialog is very cumbersome, and it doesn't explain why it's a bad idea, or how it can be resolved.
Limi proposes to streamline the installation process for Firefox on the Mac by using an installer much in the same way Apple often does. You download the Firefox disk image, it gets mounted automatically by Safari, and the installer launches by itself too. The installation routine is similar to that of Windows applications, and as an added bonus, allows users to set Firefox as the default browser at the end of the routine. When it's all set and done, Firefox is launched automatically.
Limi adds a small note that advanced Mac OS X users will be able to interrupt the process at any given time, and drag the application to the desired folder using the traditional installation method.
I'm not so sure if this is a desirable solution, and John Gruber agrees with me. He acknowledges the problems with disk images. He claims it stems from the rather abstract concept of a virtual volume, something many users simply do not grasp - and should not have to grasp. Gruber proposes a much more elegant - and old-fashioned - solution.
Back in the olden days, applications for the Mac were delivered via StuffIt archives, a compressed package containing the application in question. This method is seeing somewhat of a resurgence on the Mac: download a zip file containing the .app bundle, expand it, and have fun. This way, you can run the application right away, whether it is located on the desktop or the downloads folder, without any penalties.
An additional advantage is that it can be dragged to the dock right away, without running into the problem of the application residing within a disk image. On top of all this, Gruber proposes a mechanism by which when you launch the application for the first time, a dialog asks you if you want to move it to the ~/Applications folder.
Gruber's solution (which is actually an old solution repackaged) seems like the ideal way to go for the Mac, as it solves all of the disk image problems without really introducing new ones.