When you submit a single-word search, Firefox checks whether it could be a local address (e.g., http://intranet/). If the DNS server reports that it is a valid server, Firefox will display that bar.

There's a good chance you have "dishonest DNS" by which I mean, a service that reports to Firefox that every word you search is a website address, so it can show you a page of advertising.

If this is a "service" of your ISP, you often can opt out of it. Try going to a nonexistent address like:

http://some.crapimadeup.com/

If you get a page of advertising instead of server not found, check around for a link on the right or toward the bottom similar to:

• about this page
• opt out

Also, check for potential problem add-ons. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any extensions associated with Firefox features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious, that you don't remember why you installed it? If in doubt, disable.

Any improvement?

It worked. The dns had an opt out that I used, and now the problem is gone. Thanks for the quick answer