For bugs in Firefox Desktop, the Mozilla Foundation's web browser. For Firefox user interface issues in menus, developer tools, bookmarks, location bar, and preferences. Many Firefox bugs will either be filed here or in the Core product. (more info)
At present, the autocomplete widget can be accessed using the down arrow key.
Apparently IE allows for the use of a double-click to do the same (see the URL).
There may be a cross-platform issue here, but since I'm using Linux I'm not
aware of one.
Related Mozilla bugs: bug 48982 and bug 63320