This addon for Firefox for Android sends a Chrome user agent when visiting the Google search domains. It's useful because Google serves a "lite" version of their mobile page to non-webkit browsers. It seems that Firefox can handle the "full" version of the site well enough that the extra functionality is beneficial.
It's available from the Mozilla addon site here: https://addons.mozilla.org/en-US/firefox/addon/chrome-ua-on-google-android/
This is the User Agent used:
Mozilla/5.0 (Linux; Android 6.0.1; SM-G928F Build/MMB29K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Mobile Safari/537.36
This addon is based on user-agent-rewriter from:
https://github.com/mdn/webextensions-examples/tree/master/user-agent-rewriter
It's based on the new Webextensions standard.