About:Config Tweak
To adjust the built-in Ctrl+Enter setting, you can just set some values in the Firefox configuration to change the default from .com to whatever else you want. If you want to change Shift+Ctrl+Enter then you’ll have to skip down to the next section.Navigate to about:config in your address bar, and then filter by browser.fixup as shown here:
There are a couple of values that you can set related to this setting:
- browser.fixup.alternate.enabled – You can use this to turn the Ctrl+Enter behavior off entirely.
- browser.fixup.alternate.prefix – This value sets the prefix assigned before the keyword you type.
- browser.fixup.alternate.suffix – This value sets what is assigned after the keyword. For instance, you could use this to change the default suffix to your country code, like .co.uk
Use the URL Suffix Extension
If you want to configure all of the settings, including the Shift+Enter for .net and the Ctrl+Shift+Enter for .org, you can use the URL Suffix extension to configure all of them, or even set a custom value for when you hit Enter in the address bar.Install the extension, and then open up the options through the add-ons screen. You’ll see this easy to use dialog:
Any changes you make in here should be immediate.
Source:-http://firefox-technical-support.blogspot.in/2011/07/change-ctrlenter-behavior-in-firefox.html