Inspect running extensions in Browser Toolbox; Browser Content Toolbox; Browser Console?

Is it possible to inspect a running extension’s visible UX, (HTML and CSS)?

Perhaps from Tools > Web Developer > Browser Toolbox ?

I tried, on the Sage-Like extension in the Sidebar but could not get it to work. Even tried switching documents with the top right drop menu though nothing there looked like it was related to the Sage-Like extension.

Just right click and “Inspect element”. And the proper console will open.
Except for pop-ups, there it’s a bit complicated.