Mouse operation will be impossible.
Error console show:
TypeError: this._targetActor is null 95 node-picker.js:57:24

Steps To Reproduce:

1. Start Nightly
2. Open Library window(Ctrl+Shift+O)
3. Open Browser Toolbox(Ctrl+Shift+Alt+I)
4. Click Element picker at the top-left, but don't pick any of the elements.
5. Click Target document icon at the top-right and Select chrome://browser/content/places/places.xhtml
6. Click the element picker icon a few times to turn off the element picking mode
7. Try mouse clicking on the browser.

Actual Results:
Mouse click no longer works.
Left/Middle/Right mouse click stops working on Browser UI.
Closing the browser toolbox will not solve the problem.

Error console shows:
TypeError: this._targetActor is null 95 node-picker.js:57:24

Expected Results:
Mouse click should work.

Reproduced the issue on macos, thanks for filing!

Verified on current ESR, with the old Browser Toolbox highlighter implementation. The issue already happens there, and is even worse because you can't recover from it. Whereas on current central, if you switch back to the original browser.xhtml frame, you can interact with the browser again.

So definitely an old issue. This might get fixed automatically when we start treating frame switching as target switching (Bug 1608054). Right now frame switching is a very special case which is not well tested. We probably don't cleanup the highlighter state when switching to frame.

But we might also attempt to fix it earlier, so not strictly blocking this on Bug 1608054.