1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. Locate the preference keyword.URL, right-click on it and choose Reset

Also see the Location bar search article.

Can't believe it was that easy, and that I wasn't able to find this solution before. Not sure what caused it in the first place, but TonyE, you rock. Many thanks!

I am having the same problem, but when I can't recent that preference: when I right-click for the contextual menu, "reset" is grayed out. The value in the keyword.url preference is http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q= But not getting a Google result when I type something in the location bar - I'm getting that useless searchassist.com result. Any ideas?

See http://en.wikipedia.org/wiki/DNS_hijacking

You may have an ISP that automatically redirects you to a search page if a DNS look up fails.
That takes the control from Firefox to do the normal keyword search like a Google I'm Lucky search.
In such cases you need to contact your ISP and ask for a DNS server to opt-out of that feature.
You can also check if there is a link on the search results page to go to your ISP and opt-out.

I had the same peoblem and i found that if you are using DynDNS Updater, go to Advanced settings and disable the DynDNS(dot)com Internet Guide tool

I found that clicking View/toolbars/customize/add toolbar then entering "location bar" in the blank space put the location bar (address bar) back into the view/toolbars menu.