This pref doesn't exist by default.
It is possible that this pref has been added by the user.js file in the profile folder.

Your System Details List shows that you have a user.js file in the profile folder to initialize some prefs on each start of Firefox.

The user.js file is only present if you or other software has created it, so normally it wouldn't be there. You may want to check its content with a plain text editor if you didn't create this file yourself.

The user.js file is read each time you start Firefox and initializes preferences to the value specified in this file, so involved preferences can only be changed temporarily for the current session.

Can you list the content of the user.js file?

That's all it says. Do you know what it does? If you do I might be able to figure out what changed it and/or if I'm able to fix it.

http://kb.mozillazine.org/Network.protocol-handler.warn-external-default

See these - looks like Malware on your PC probably created that user.js file with that preference.
http://forums.malwarebytes.org/index.php?showtopic=117172
http://www.bleepingcomputer.com/forums/t/480723/skyware-or-malware-infection/

Install, update, and run these programs in this order. They are listed in order of efficacy.
(Not all programs detect the same Malware, so you may need to run them all to solve your problem.)
These programs are all free for personal use, but some have limited functionality in the "free mode" - but those are features you really don't need to find and remove the problem that you have.
Note: If your Malware infection is bad enough and you are mis-directed to URL's other than what is posted, you may have to use a different PC to download these programs and use a USB stick to transfer them to the afflicted PC.

Malwarebytes' Anti-Malware - http://www.malwarebytes.org/mbam.php
SuperAntispyware - http://www.superantispyware.com/
AdAware - http://www.lavasoftusa.com/software/adaware/
Spybot Search & Destroy - http://www.safer-networking.org/en/index.html
Windows Defender: Home Page - http://windows.microsoft.com/en-US/wi.../windows-defender
Also, if you have a search engine re-direct problem, see this:
http://deletemalware.blogspot.com/2010/02/remove-google-redirect-virus.html

The file is clean, and I don't have a problem with being misdirected to other URLs. Could it be because I downloaded a torrent client a while ago and set it from the software itself to open magnet links in Firefox?

Did you check the content of the user.js file in the Firefox profile folder with a text editor (e.g. Notepad)?

Your System Details List shows hat such a file exists.

You can check its file data to see when it has been created.

Yeah I did, I mentioned that that's all that's in the file in my first reply. There's nothing else, just what I quoted in my original question.

Hi: The original person who posted this problem has the same problem that I have. My firefox is going crazy and I'm not sure why. I have the same user.js.file problem. When I clicked on the link, it has lots of information but it's from babylon and funmoods, etc. I tried to select all and delete it but it won't. In the meantime, my firefox is "not responding" like every few minutes. I'm not sure if it's an update from java, shockwave, or flash player. I uninstalled the shockwave and it hasn't helped. I keep getting these messages saying I need to update my java but I've checked on it and it says it's uptodate. I tried clicking on this java update and it downloaded other things with it. I've been uninstalling them but since then my firefox has gone beserk. I don't know how to fix it. Any ideas?

Hi 1mercury, you'll need to do a lot of clean-up of unwanted programs in both the Windows Control Panel (I assume you run Windows) and on Firefox's Add-ons page. To get specific assistance with that, please start a new thread and include your Firefox configuration information.

You can start a new question at the following link. Scroll down past the suggestions to continue with the form (I think it has 3 different screens). https://support.mozilla.org/en-US/questions/new/desktop/fix-problems

You can create a new profile if your current profile is causing the problems that you can't fix easily.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox