Ah, I’m sorry, I thought you could use any Firefox add on in the mobile app but it looks like it’s limited a small list that doesn’t include a user agent switcher. Sorry for the misleading suggestion! Another option would be to install the Firefox Nightly version, it’s the newest version of Firefox mobile available and hopefully they’ve solved your issue in that version!
You could also try using a general user agent spoofer and set it to spoof to chrome mobile when visiting google.com until the search fixer add on is fixed