I've found that it's pretty normal for the Firefox window to appear all white when you first start it, while Firefox actually loads the browser elements. Depending on how fast your system can load the Firefox elements, will determine whether or not that white screen is noticeable.

I've worked on very slow systems, in which the Firefox window is white for almost 5 seconds. Alternatively, my system only shows the window white for a fraction of a second.

Hi Wesley, Thanks for your replay. De white screen only shows for a fraction of a second. But it suddenly started last Thursday. There was no change in any software on our computer as far as I know. So I wonder what started it.

It could have been a Firefox update that marginally slowed performance or something else (Windows Update, security software, hardware wear, etc.). There are tons of factors that could cause this.