Some clarity
Ok let's make a few things clear...
When attempting to open a .torrent file, the application that is used to lanuch the .torrent file usually determines what application it is opened with.
Commonly used applications to launch torrent files are:
File Browsers - Explorer (Windows), Finder (OSX), Nautilus (Gnome), Konqueror (KDE), etc. (You need to remember that although these applications are commonly bundeled with their respective Operating Systems; they are indeed applications.)
Web Browsers - Firefox, Chrome, Opera, IE, etc.
Each of these applications can be independently configured to open torrent files with any application. Most users will only have one torrent application, and most other applications on the system will be configured to use this sole torrent application to open torrent files. However this is not always the scenario.
GGJ - You are correct that torrent files are usually opened with standard torrent applications. But you are incorrect that these files cannot be opened with firefox. With the correct torrent addon, this is entirely possible. (I know ultimately this a small program running inside of firefox that handles the downloading of the torrent. But this is seamless to the end user. And an end user would say that they "opened" the file in Firefox.)
Also nobody on this thread asked about automatically opening files after they have been downloaded. What the Gentlemen who started this thread was trying to ask was:
Is it possible to configure firefox not to display the firefox open file dialog, when a file is launched from the firefox download manager, an application is selected to "open with", and the "Do this automatically for files like this from now on" option is checked. For whatever reason, Firefox seems to ignore that this option is enabled when handling torrent files.
Also I wanted to state that I am not a regular user on this forum, and that I registered just to reply to this thread. When I stumbled upon this thread on saw that a such a simple question was answered with such arrogance and ignorance, I was motivated to reply. I felt that the author of the thread was looking for a solution to his problem, not a lesson in semantics. Try shaping your responses to be a little more constructive, rather than destructive, as this helps no one.