Sure, since these features are just written in HTML and JS (the modern Firefox devtools use HTML, not XUL), they could certainly be downloaded on demand. The ad

Sure, since these features are just written in HTML and JS (the modern Firefox devtools use HTML, not XUL), they could certainly be downloaded on demand. The added complexity of that combined with the relatively small size of the code that implements the the UI means that there's not much reason to do so. And it's only the UI that could be downloaded on demand; it'd be pretty hard to break the JS engine in half to put breakpoints into an extension, or to ruin the layout engine so that you need an extension use the CSSOM.