Omnibar is an Add-on that allows you to use the address bar as a search bar, as well.

Install Omnibar and (assuming you use Google as your default search engine) type a search term in to the bar and hit enter. You will now be taken to the Google site, displaying the search results.

Welcome to Firefox. The address bar will use the same search engine selected in the Search bar (by default, it appears to the right of the address bar, and by default, it's Google). You can use Ctrl+L to move the focus from the page to the address bar, but to submit your search, just press Enter. Or press Alt+Enter to open the results in a new tab.

Note that if your query is a single "word" (e.g., intranet, or mozilla.org, basically, no spaces), Firefox will first check whether the word is a valid host name. Some DNS providers (typically your ISP) may return its own search page instead of site not found. Some ISPs allow you to opt out of this or, if not, you could change your DNS servers (more help on that if needed).

More info: Search the web from the Address Bar

Here's the error that I get The DNS server returned:

   Name Error: The domain name does not exist.

Hi ranveeraggarwal, could you run this search reset extension and see whether the bar behaves any better?

https://addons.mozilla.org/en-US/firefox/addon/searchreset/

If the DNS query is returning that error as a page (code 200) instead of a not found (code 404), then Firefox won't recognize that it needs to search. You can view the response code in Firefox's web console. Press Ctrl+Shift+k or open it from Tools > Web Developer. Then try the search again and see what Firefox shows after the URL of the request.