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1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise!, to continue to the about:config page.

change browser.urlbar.autocomplete.enabled to false 2 right click menu and select customize

Actually what you want to do is follow the same procedure but set

browser.urlbar.autoFill to false

That stops the url bar filling bit keeps the dropdown menu.

See also "Prevent Firefox from automatically completing URLs":

• https://support.mozilla.org/kb/search-your-bookmarks-history-and-tabs-awesome-bar

Actually, I can't use about.config because the auto complete adds "www," making it "www.about.config" !!

It is about:config with a colon (':') (it is a protocol like http) and not a dot ('.')

• http://kb.mozillazine.org/about:config