This looks like a change made by Google. If you right-click that input field and open the Inspector then you will notice that this is a textarea with one row (rows="1"). However, when I press the Enter key then I submit the form, Shift+Enter gives a linefeed. If that isn't working for you then you may be blocking JavaScript.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

@cor-el

Thanks for your advice.

Using the Refresh Firefox feature fixed the problem.

(It also removed all Firefox extensions.)