The solution is actually very easy. It has nothing to do with Firefox, one of your security tools is messing with Firefox.
1. Step Click on the orange Firefox button and click on Add-ons
2. Step Click on the Disable button of AVG Safe Search 12.0.xx
3. Step Restart Firefox
4. Step The Enter, return key will now work again.
Optionally, if you’d rather not want to disable the AVG Safe Search extension, you can simply use the small arrow that appears when you enter an URL into the address bar to submit the URL and load the website.
Did you notice that any other keys beside the Enter/return key do not work? Let us know!