Firefox allow users to specify a search engine that has to be used for search. This can be done using the Search field available at the top as show below.
The default search engine used by Firefox fox any searches is Firefox but if you want to add a new search engine then you can use the Manage Search engines to do this. For example if you want make Bing as a default search engine in Firefox, then click the drop down arrow in the search field which would display the already installed search engines in Firefox.
Click the Manage Search Engines from the list to open Search Tools :: Add-ons for Firefox webpage. In the search tools page, type Bing and click the search button.
And select Bing from the search results by clicking the Add to Firefox button.
In the above example, we are adding Google + Bing search engine. The Following Add Search Engine message will be displayed.
The Search Field in Firefox will reflect the addition of this new search engine as shown below