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

Have you even checked the Proxy Servers tab?? When you are adding servers there is a BIG box for username and password which obviously are for authentication of that proxy server!

Edit: Yes i'm the developer and it clearly says that.
1- The screenshot you are referring to is a limitation of WebExtensions API and browser provides and I cannot do anything about it. AND that is an EXTRA FEATURE for advanced users .
2- The thing you want to achieve is the goal of the proxy if USER CONFIGURE IT.
The SmartMode is for that purpose. First, you need to ADD the domain you want to apply proxy for (in Rules tab).
Second, add proxy-servers. Third, put the extension in SmartMode.