Firefox add-on RestClient says Chrome://restclient/content/ how?

I have been using RestClient on OSX's firefox browser. All of sudden I have noticed the address-bar of the Firefox. Please see the indication in the image.

I want to know the reason for this & how to resolve it to normal ?

XML User Interface Language (XUL) is used for building (among other things) Mozilla's apps and add-ons/extensions, and locally running XUL apps are sometimes referred to as chrome (the word can also be used to refer to UI/window elements such as window frames, menus, toolbars and scroll bars).

There are a whole bunch of default chrome:// URLs defined for various Mozilla apps; you can check out the list here.

What was the URL shown before the change? As per the screenshots on the official add-on page (see one of them below), the chrome:// URL seems to be pretty much the same as in your image above. Does the RESTClient add-on still work properly? If so, why worry?