According to the plans from the wiki  we are going to implement a new View Source component that is reusable within other developer tools. We will also develop a new UI for a new View Source tool for Firefox that will go beyond the current View Source tool. This bug holds the user interface implementation of the new View Source tool. Please note that the first take is going to be minimal - we are going to only attempt an implementation that is as powerful as the current View Source tool. For additional features we will use separate bugs and patches.  https://wiki.mozilla.org/DevTools/Features/ViewSource
Change of scope. We will probably use both the new Source Editor component and the current View Source implementation - a hybrid solution. We'll open new bugs as needed.