I've noticed when downloading programs from some sites that the file often doesn't show the proper name, but shows the name of the Perl script which initiated the download. (eg, filename.pl). This usually shows as a text file or generic document.
Experimenting by renaming the .pl extension to either .sit or .dmg, usually solves the problem as downloads are usually Stuffit files or Disk Images.
[Editor's note: I haven't noticed this problem in Mozilla...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020318014502509