Firefox browser allows users to use the address bar for both search and navigation. You can enter an address to visit a site or type some text into this bar and do a quick search with your favorite search engine, Google, Bing, Yahoo, DuckDuckGo, etc. Google is the default search engine in Firefox browser. The browser allows you to add or remove search engines, change your default search engine. If you are using Firefox for mobile on an Android device, here’re the steps to change the default search engine.
Set new default search engine in Firefox
Launch Firefox browser on your mobile phone or tablet. Tap the Menu button (three vertical dots) at the bottom right corner, then select Settings from the pop-up menu.

Tap Search on the Settings screen, a list of search engines displays. Find your preferred search engine and tap on the checkbox in front of it to set it as the new default search engine.

Add new search engine to Firefox
If you like to add extra search engine to the list, tap Add search engine below the list. And then tap the Other radio button. Give a name to the new search engine. Type in the search string to use and replace query with “%s”. If you don’t know which search string to use for a particular search engine, simply access its website in a new tab and do a search. Here we’ll use yandex.com as an example. If you go to the home page of this search engine and search for “apple“, the resulting URL in the address bar will be something like https://yandex.com/search/?text=apple&lr=xxxxx. Then the search string should be https://yandex.com/search/?text=%s.

Tap the confirmation button at the top right corner of the screen. Then the new search engine will be added to the search engines list.
Touch the radio button before the search engine you have just added and set it as the new default search engine in Firefox browser for Android.