I've been frustrated for a long time by the fact that when using
www.google.com to search, when entering search terms I get a new screen with my search terms at the top and the text, "Press Enter to search." but pressing enter is ignored. Clicking the blue search button works. Well I finaly had enough and tried searching this problem. There are tons of comments from 2009 but this is 2015 and the versions of Firefox and other possible software conflicts mentyoned have changed so much, I haven't gotten anywhere with this. If I turn off instant predictions, I stay on the main screen when typing but enter still does not initiate a search.
I have Firefox 38.0.1, some plugins and add-ons, Avast AV and not sure what else to mention.
IE 11 works fine but I try not to use that.
Is this something recent? If so run a System Restore to a time before the problem started.
Have you rebooted the PC recently?
Uninstall/Reinstall FF
I understand the Enter key works OK in IE.