There are pros and cons to using web tech for the application UI. To be clear, the old XBL code was already using JS/DOM, so the most obvious pro in this case was that it didn't require completely rewriting Firefox from scratch (which hasn't historically worked out well). I also expect that dogfooding web technologies means we can improve the tooling and web standard implementations in Gecko.

Regarding performance, this work didn't regress anything. Performance is measured on every commit and staying at least at parity was a requirement for the project.