By typing a non URL text in the address bar and pressing <ENTER> the browser should automate search with the default search engine. Else one has to click two more times: the OK button and then the SEARCH button.

This works if you have internet keywords enabled. And according to http://www.mozilla.org/docs/end-user/internet-keywords.html you can even set a pref to use (for example) google as your default internet keywords search. All the various bugs on this make for a very confusing mass though. Resolving as a dupe of another bug this actually is a dupe of. *** This bug has been marked as a duplicate of 123968 ***