Creating and Deploying Update Packages

This is what I do:

1. Download Firefox or update the current version that I have to the latest.
2. If downloaded, I launch and install it. It normally ask to drag the installer to the Application folder.
3. launch it few times to make sure it doesn't ask to be the default app and so on.
4. make sure I remove the installer file (DMG that I downloaded from the web)
5. Launch Composer
6. Navigate to the Application folder using Finder.
7. Drag and drop Firefox in to Composer.
8. Build as a DMG or PKG
9. Drag that in to Jamf Admin
10. After I index it properly, I save Jamf Admin.
11. Create a Policy and deploy to test systems.
12. If successful, then deploy it to everyone or specific users.

That's what I do and it works.

As detailed above, you can't use DMG files from vendors "as is" in Jamf Pro. This is a common misconception a lot of newcomers run into. Those DMGs usually have the .app at the top level of the mounted disk image and when a DMG is deployed through Jamf Pro, it uses the folder structure to determine where the contents should go on the destination disk, which is why it's ending up where it does - the .app is at the root of the mounted disk, so it ends up in the root of the drive.
You need to either go through the process outlined above, or use a script/utility to take those DMGs and create valid .PKG installers from the included app bundle in them.

Here are 2 examples of such utilities:
Rich Trouton's Simple Package Creator
My App Packager

There may be others, but they are likely all doing the same general thing under the hood.

+1 for App Packager, have used it for years

The best ideas are above but I’ll add some background.

A “Jamf-friendly” dmg file should ultimately be a mirror of the Mac’s file tree for your additions. For example if you are adding a script to the Scripts folder in Library, I would expect to see a Library folder inside your dmg with a Scripts folder inside. Inside that Scripts folder would be the script itself that you wish to add. Same for an app in Applications.

A pkg is similar but it (unlike a dmg) would allow you to run a script before or after the installation of your package and it would work if you just gave it directly to an end user to install.