I run Safari on my iMac, but I find Firefox to be a bit quicker and a little more lightweight so i run it on my much slower Tangerine iBook. Safari properly handles the .SMIL files that npr.org sends to allow me to listen to Real Audio, but Firefox just saves the file. When I attempted to select RealOne Player as the helper app to launch for .smil files, it was greyed out. All of the applications were greyed out. Data files were selectable, but apps weren't. Searching for help out on the net, I found this bug report on Mozilla.org. Seems that files ending in .app aren't coded to be recognized as apps. Oops.
That got me thinking: what if I dug into the .app package contents and selected the actual embedded RealOne Player executable file? In the Finder, I control-selected the RealOne Player executable and did a Show Package Contents from the contextual popup menu. In the new Finder window that popped up, I navigated into the Contents folder. Then I Option-Command dragged the MacOS folder onto my desktop, creating an alias on the desktop. Then, back in Firefox, I was able to open the Prefs, select the Download options, and change the action on the SMI type by selecting the application by navigating through the desktop alias to the RealOne Player application.
Now Firefox launches RealOne Player for .SMIL files just like Safari does. This should be applicable to any other MIME filetype handler applcation also.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040324200720733