We have this :-moz-styleeditor-transitioning rule here and here (why in two places? I don't know).

It's intended to provide a transition for every node in the document while you're editing styles, but it's problematic due to a number of reasons. The biggest one is that it overrides the computed transition values from the page. So you can't actually use the style editor to code any CSS transition, because we override it.

The other issue is that even for medium pages (I was testing searchfox, but...) the work updating the page takes long enough the transition is unnoticeable.

Another issue is that that selector doesn't work for shadow DOM of course.

We discussed removing this with the team, but there was some concerns, as this is something other vendors DevTools don't have.
We might think of ways to keep the feature without having this specific stylesheet