When recent versions of Firefox (currently on 111) start, they first open a weird blank-model view, of apparently random size, before that view disappears and the actual browser loads. It's janky and just not a good design.
While I appreciate that we want to give the appearance that Firefox is starting more quickly - show the users something while it's loading from disk - this visual treatment is bad design. The kind of "show a blank model with gray boxes" design as a loader is good for web sites themselves, but not for the browser. If you want to indicate that Firefox is getting started, it's better to show an application modal (NOT system modal) small dialog, and then have that dialog go away when the browser is ready.
The current design makes your eyes jump all over the screen, not knowing when or where I'll be able to click on something. Compare that to the experience of starting a Microsoft Office application; there's a small dialog in the middle of the screen that's clearly not the application, and then it disappears when the app is ready.