I have a strange firefox problem in my Netrunner 17 Ubuntu 16.04: I’m using Firefox 1000~50.0.2+build2-0+8+build39 vom the plasmazilla repository and the address bar does not accept the enter keys anymore - so I cannot launch any website unless it is in my favorites or I use the search engine from the start page. The Enter keys do work in other applications, e.g. chromium.
Strange indeed. Can you try to create a new mozilla profile and see if it works with this.