I'm just guessing here, but I think you may be seeing a web page on your desktop which replaces the Windows Active Desktop. I've had this happen to me a few times.

To test it, right click anywhere on the white screen, go to Properties | Desktop | Customize Desktop | Web (tab) and then delete any web pages which are in there. Then Apply/OK.

A possible cause is security software (firewall) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process.

See:

• Server Not Found - Troubleshoot connection problems
• Configure firewalls so that Firefox can access the Internet

Has the problem been resolved, or is it still ongoing?

Well, I alternate between Windows and Linux. I'm currently using Linux, and I'm not having the problem here, at least not recently. Next time I'm in Windows, I'll see if I still have the problem.