(Note that browser console/BC in comment 0 should be Browser Toolbox/BT)

I can still reproduce this.
We're hitting this line: https://searchfox.org/mozilla-central/rev/22a9cf9388cefa244ea2558efa748ce17a38d607/devtools/client/fronts/inspector.js#157-160 , which is a result of https://searchfox.org/mozilla-central/rev/22a9cf9388cefa244ea2558efa748ce17a38d607/devtools/server/actors/highlighters.js#87-89
Simply removing https://searchfox.org/mozilla-central/rev/22a9cf9388cefa244ea2558efa748ce17a38d607/devtools/server/actors/highlighters.js#72-76 makes the grid highlighter (and the flex one) just fine.

In order to fix this bug, we could in a first time just add a XULSupported getter like in other highlighters (e.g. https://searchfox.org/mozilla-central/rev/22a9cf9388cefa244ea2558efa748ce17a38d607/devtools/server/actors/highlighters/accessible.js#97-99). It would be nice to see if we still have the limitation around XUL window, and see if we can simply remove the condition

I wasn't able to retrieve the original STR this condition was preventing, but
removing it and using various highlighters (box model, flex, grid, …) in the browser
toolbox, targetting various documents, I didnt' see any error.
Most importantly, it allowed to show highlighters where it wasn't possible before
(e.g. grid highlighter for webconsole document).
TRY don't show any breakage either.

https://hg.mozilla.org/mozilla-central/rev/c675dbfff651