When I click “Inspect” the new debugging system loads in a tab ready for debugging the background page.
Previously I was able to select the “Disable Popup Auto-Hide” option, click to open the popup and then select the popup iframe in devtools.
When going through those same steps in FF68 and 69, the iframe chooser popup never closes and the devtools mostly freeze (switching tools works but everything else appears to be blocked, possibly because this iframe popup never closes). It used to close in the old non-tab-based debugger tools, even when the auto-hide was disabled.
This same limitation seems to happen whether I keep the devtools tab in the same window or tear it out into a new one (as is now required every time in order to be able to actually see the popup and devtools at the same time).
Pressing escape does not force the iframe picker popup to close.
I find it hard to believe that no-one else has tried debugging a popup since FF68 hit Nightly so I guess I must be missing something?