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I understood the question from the beginning!...stop the bickering!! I googled the problem and this thread was at the top of the list! There was another thead which steared me to the solution. This is the fix for firefox:

Tools>Options>Applications>TORRENT File-->Use uTorrent(default)

OK, it's an old thread and I'm a Windows dude but I joined this forum just to say Thank You to jrloader, this has been driving me round the bend for ages - your fix worked a treat.

Once upon a time I would click on a torrent link and it would fire up automatically in uTorrent. Somewhere along the line that got screwed up and for months after that when I clicked on a torrent link the torrent file was downloaded to my desktop and I had to open uTorrent and navigate to the torrent file to open it manually. Now I'm 'back to normal' so Thanks friend :)
 
I know this is old, but I just resolved this issue for myself. This is for those of you that are experiencing the following problems with Firefox: Open Menu > Options > Applications-- Under Content Type, your file extension does not appear. Correct? Resolving this is very simple! You have to follow the steps listed here on the Firefox Support Site. Now, I'm sure you just got a little frustrated realizing that you've seen this exact site telling you to do what you've already tried a thouand times, and the option to "Do this automatically for files like this from now on" is greyed out! So, get this. Now it gets even easier! You're going to want to follow this link and download the sample torrent file. The option to Open With, and your selected program, should of course be available anyway; and you should be able to chack the box to automatically open with your selected program. Tada! If your issue is that the program you want to use won't appear even after browsing directly to it's directory and selecting it, LET ME KNOW!!! I will resolve that as well.
 
Ok, so for those of you Windows users that got lost here, in MacWorld, that are having the issue with your 'Open With' dialog,this is another quick fix. Now don't bypass this just because you see the words 'win xp', because I tested it on Windows 10 and it passed with flying colors. You want to follow this link, and choose the Hyper-Link "OpenWithAdd.zip", to download a program which is an extremely quick-fix to this issue. You want to save time, eh? Click this link, and your download should begin automatically. Run the program from the zip file and select the directory to the program you wish to be displayed and available in the 'Open With' dialog, or enter the directory manually (eg. C:\Users\owner\AppData\Roaming\BitTorrent\BitTorrent.exe); leave the [Options] spaces blank if you don't know what you're doing (advanced users, have at it! :D), and select 'Register'. A dialog box should appear reading "Done!". Great Job, now go see if it worked. If it didn't, Reboot your PC and you should be good to go!
 
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