How do you set the default search engine in firefox?
Figured this out by looking at the desktop version.

Enter "about:config" into the address bar.
Search for "keyword.url"
Set the value of this setting to the new search provider. For example: "https://www.duckduckgo.com/?q="

UPDATE:
This has changed in the newer versions of Firefox (desktop version too).
You now have to install search engine addons.
Then enter "about:addons" in the address bar.
Long press on the search engine add on that you wish to be default and select "Set as Default" from the menu that appears.