Firefox for Macintosh OS X may not download DMG (Macintosh Disk Image) files so you can easily open them. When you double-click a DMG file after downloading it with Firefox, it tries to open with a program called "HexEdit". You can mount and open this kind of DMG file if you know the "trick". The following explains how to do it.
When you download a DMG file using Firefox in Macintosh OS X you might see an icon similar to the one shown above. Firefox sees the DMG as a generic binary file, but it doesn't necessarily know what application to open it with, and it might need a little help.
To get this type of DMG file to open, do the following:
STEP 1: Right-Click, or Control + Click on the DMG file, once it is fully downloaded. You will see a menu like this:
STEP 2:Choose the "Open With" menu item, then "DiskImageMounter". After a few moments, depending on the size of the DMG file, you will see the mounted Disk Image show up on your Desktop.