If you're using Safari, option-click this link, then just remove the .html from the end of the .dmg it downloads. Their server is treating the dmg as an html file for some reason, and it has been for a few versions now.
http://download.mozilla.org/?product=firefox-1.5&os=osx&lang=en-US
[Version 1.5]
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SNES9X
Berkleebassist rated on 03 Apr 2011
[Version 1.52]
RipIt
Berkleebassist rated on 03 Apr 2011
[Version 1.5.5]
SymbolicLinker
Berkleebassist rated on 03 Apr 2011
[Version 2.0v2]
Mac Face
berkleebassist reviewed on 25 Jun 2007
Tangerine!
Firefox
http://download.mozilla.org/?product=firefox-1.5&os=osx&lang=en-US