Why is this dialog being displayed when I start firefox?
Solution 1:
This isn't a dialog, this is the result of mounting (opening) the Firefox.dmg (think of it as an archive or drive image). For mac users it's obvious - you should drag the FF icon to the Applications folder shortcut (the blue folder). This will copy the Firefox application in the /Applications/ folder. Then just unmount (eject) the Firefox.dmg archive and start Firefox from Applications (like you start the rest of your applications).
Solution 2:
The file what you downloaded, is an "DMG" archive type (like ISO image).
- When you double click on it, you open it (called mounted it).
- After opening the archive you should install the application - simple way - drag and drop into the destination.
- After the install - eject and usually can trash the dmg archive.