White Box in the Middle of My Screen

Is it still there when you change the background picture?

Does it happen in Safe mode?

OS X: What is Safe Boot, Safe Mode?

Have you rebooted?

What model Mac Pro? What graphics card?

If you take a screen shot does it appear in the screen shot?

If it does not appear in a screen shot that would tend to indicate a graphics card problem.

Do you have a bootable clone yon can boot from and try?

Please read this whole message before doing anything.

This procedure is a diagnostic test. It won’t solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of this test is to determine whether the problem is localized to your user account. Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”

While logged in as Guest, you won’t have access to any of your documents or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this behavior; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.

Test while logged in as Guest. Same problem?

After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.

*Note: If you’ve activated “Find My Mac” or FileVault in OS X 10.7 or later, then you can’t enable the Guest account. The "Guest User" login created by "Find My Mac" is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.

If Safari is not affected, you may have installed a malicious Firefox extension.

First follow the instructions on this page. If there's a Firefox extension you can't remove in the Extensions manager, see below.

Back up all data before proceeding.

Triple-click anywhere in the line below on this page to select it:

~/Library/Application Support/Mozilla

Copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select

Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.

A folder should open with an item named "Mozilla" selected. Quit Firefox if it's running. Move the selected item to the Trash. Relaunch the browser and test.

If the extension is still present, repeat with this line:

/Library/Application Support/Mozilla

If this folder exists, you may be prompted for your administrator login password when making changes to it.

Some kinds of malware may insert code into the Firefox application itself. If the above steps don't solve the problem, quit Firefox, delete it from the Applications folder, and download a fresh copy directly from mozilla.org.

It might help others to know which Firefox extension caused the problem.

OK, thanks.