Firefox Nightly shows a blank window with legacy default UI styling before loading the current new UI on startup

[RavenVI]

I am using Firefox Nightly version 155.0a1 on Windows 11 24H2.

On every cold start, Firefox briefly displays a blank window whose default styling matches the legacy UI rather than the current new UI shipped in this build. After a short moment, it correctly switches to the new UI.

Please let me know if any further logs (about:support, startup logs) are needed. 🙂