To switch search engines, click the down arrow next to the current search engine icon and select a new one. I think it will help u :)

There apparently is not an down arrow on the Google Nexus 7 Firefox web browser. I've tried everything I can to replace Yahoo as the search engine with what I want with no luck on this device.

hello, you're right - there is currently no user-interface to set or change the default search provider for firefox on android. this will come in one of the next versions though (likely in firefox 23), then you'd be able to change that in the addons menu for a selected search engine - here's a screenshot on how it will look like: https://bug730445.bugzilla.mozilla.org/attachment.cgi?id=726425

Aurora (Firefox 23) currently has this functionality with selecting a default engine in the Add-ons Manager.