Hi

Which distro and desktop environment are you using?

Are you using a distro version of Firefox, or did you download it directly from Mozilla?

What are the search settings for the location/address bar?

If they are all disabled then this might cause the Enter key not working.

The location bar drop-down list can show history items, bookmarked pages, pages opened in tabs, and search suggestions. You can set what suggestions show When you enter an URL in the location/address bar bar via Options/Preferences -> Privacy & Security

• Address Bar: When using the location bar, suggest:
  History
  Bookmarks
  Open tabs