User-Agent: Mozilla/5.0 (Mobile; rv:44.0) Gecko/44.0 Firefox/44.0 There's no enter key on the keyboard, only search and backspace. I couldn't find any way to go to the URL directly.
The search icon in the keyboard is the "go" button for the URL entry as well. That's not to say this issue isn't a bug, but it will require some UI testing to uncover the best solution. Assigning to ourselves. Thanks - your UX team!
I proposed a (partial) solution in bug 1197459.