Maybe you have already figured it out. I was looking the the same answer as you, so I thought this is a good place to note something down.
You screenshot shows a problem with the firefox developer tools. When the window too narrow you are missing important information.
If you widen your window a bit then you will see a new icon appear to the left of the “…” of the developer tools:
The blue one, above the word “Changes” that looks like a layout. When you press it you get a dropdown and you can select the page that is loaded in the sidebar.
I’ve found myself having the same issue. But in my case it seems to be something that happens on Firefox Developer Edition; if I switch to the regular Firefox version this does not happen.