Just few days ago a very strange problem occurred with my Firefox. Enter key is not working in its address bar. Default behavior is that whatever you enter is searched from in your default search engine, but now nothing happens, and you have to press go to button. Then I just checked list of installation I have done in FF and found that AVG Safe Search (FF extension) is creating the issue, I disabled that and Enter Key is working fine in address bar.
So you can also try this if you are facing similar issue.
Happy Surfing !
Similar issue has been reported on Mozilla Site as well.