It uses whatever search engine you have configured in Firefox. When I tried it a minute ago on my Nexus, this defaulted to Yahoo.

This is unequivocally Mozilla code you're observing. Here's what adb logcat shows when that activity starts:

    I/ActivityManager(29188): START u0 {act=android.intent.action.ASSIST flg=0x13000000 cmp=org.mozilla.firefox/org.mozilla.search.SearchActivity (has extras)}