> Cheaper but it can fail on slow hardware where the originl panel xbl is applied asynchronously, so that the anonymous content will not be available. That's why you had to add the if (node). It works thanks to the second call make to attach done in show.

Can you please elaborate why would this fail on slow hardware ? I consulted with platform engineers about this back when I first did this change (~3 month ago) and was ensured that DOM nodes will be made available synchronosuly including anonymous ones. Only time it's not is on first load of XBL which should not be an issue
since it's loaded with rest of the firefox UI.

> Can we keep to original code that is known to work and eventually improve this in a followup?

I'm afraid it's not as easy, as it actually had timing issues and we had to wait before our custom thing loads that makes frame loaders do strange things.