How do you set the default search engine in firefox? In firefox if I type a bunch of words instead of a web address and hit go it does a google search.

You can install addons for other search engines and they appear in a list below the address bar and you can click those to perform the search on that engine instead. 

Is there a way to make one of these search add-ons the default?
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.