Can you please post your request in here to keep track of it?
https://github.com/salarcode/SmartProxy/issues

Maybe you don't have access to fonts.googleapis.com? That is troublesome and I will definitely get rid of it.

To create a proxy that is a mandatory permission required by firefox.
check here:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/proxy/onRequest
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#Host_permissions
and here is permission pattern which is for all urls, called
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Match_patterns#Examples