With the following Firefox 18.0.1 settings on MAC OS X 10.8.2 the auto fill in does not work at all:
Location Bar: History and Bookmarks
While I was browsing through the forum I found this article:
So I changed browser.urlbar.autoFill.typed to false and since then auto fill in works for webpages, bookmarks and tags. Should auto fill in not work with the option set to true??