Thanks so much for the heads up, but it looks like I can't find a magical value to use that will fool that page into showing "Google Search Fixer" on that list. They actually seem to be guessing at a device model based on the Android version number in the string, and maybe the navigator.platform value, as no matter what navigator properties I fiddle with I can only get it to say "LG Nexus 4", "Samsung Galaxy Edge S7", or "Asus Nexus 7 (2013)". I think I'll just have to jot down a note in the addon description so that other users are aware of this.

Is there something Google Search is missing or doing differently for the desktop/Windows version of Firefox, or were you hoping to simply use the addon to get the mobile/Android Search experience on Windows as well?