There isn't a config file in Extension. You could change engine lists in Search Option in Firefox if you want. In case you want to deploy your Firefox using config files. You should have a look at Firefox enterprise group policy.

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Search engine's url could not be configured there. The only way you can do is install another search engine and remove the current one. By doing this, you need to find a open search xml for that search engine and install it. Some other tools / website may help you do this. But this may be out of scope of this extension.