From MozillaZine Knowledge Base

There is a bug in some official servers that provide the Mac OS X Firefox installer that causes the file to be downloaded as "download.mozilla.org" (Bug 280980). Since the file is downloaded with the wrong extension, Mac OS X wants to open it with Quicktime or Excel rather than install it. There are two methods of solving this problem.

Method 1

Rename the "download.mozilla.org" file to "download.mozilla.dmg". Install that file.

Method 2

• Delete the "download.mozilla.org" file from your computer.
• Redownload the Firefox .dmg installer from mozilla.com (or from the 1.0.7/mac or 1.5/mac directory on the ftp site).
  If the problem persists, use a different mirror site.
• Install "Firefox 1.0.7.dmg" or "Firefox 1.5.dmg" (Note: Firefox 1.5 requires Mac OS X 10.2 or higher.)