How to prevent Firefox converting 'localhost/*' URLs into search queries?

Since upgrading from Firefox 3.6.x, my address bar has a quirk that never used to be there. When I type in, for example:

localhost/my_dir/index.php

Firefox turns it into a search query, my search engine is Google. To work around this (after forgetting I need to do this almost every time for FF), I must type in:

http://localhost/my_dir/index.php

This way, I am directed correctly to my localhost server. I don't need to do this for any other browser, and I thought this Q/A on URL trimming might change the behaviour, but it didn't.

Any ideas on how I can prevent 'localhost' from being turned into a search query?