Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

The problem seems to have resolved itself. No issues with Firefox for the last couple of days.

It could be a corrupted file was replaced. As long as it works.

Please flag your last post as Solved Problem so others will know.

I have a brand new Dell 7352 Inspiron 13 model which came with Win 10 preinstalled. I was running Firefox and Thunderbird at the latest release levels. I noticed intermittent blank white areas or completely blank pages when switching from window to window, or after awakening from the screen saver. I use a password protected screen saver, and it may have been happening after the screen saver started and then was unlocked to return to active use. If I was able to find the refresh button on the page by rolling the cursor over the general area where it should be, and hitting it, it would sometimes restore the entire page. Some areas were still blanked out, on occasion, typically near the top of the page. Stopping and starting the applications would usually/always resolve the problem, for some period of time. I read another Mozilla support thread which described a problem similar to my problem, and there was some suggestion that the problem occurred at or around the release of Firefox V39, and that prior releases seemed more stable and didn't seem to exhibit this behavior. I have since backed off to an earlier version and have had no problems since the version downgrade. I also back leveled Thunderbird to a much earlier version, and have not seen the blank white areas/pages behavior since. I have applied all Microsoft Windows updates and all the drivers are up to date, too.