Be careful, launching firefox in your DispVM-Template is not recommended, since it’ll create a permanent profile that will be subject to profile fingerprinting.
The recommended approach is to create a policies.json file in your TemplateVM that will be used to customize new firefox profiles.
I explained everything in great detail here, including adding/removing search engines: [Guide] Automatically install extensions and configure new (dispvm) hardened Firefox profiles with arkenfox user.js and policies