Version 1.4.2 of FoxTab has just been released which fixes the problem with the small white square being shown. It is not yet available when using the built-in updates, but can be obtained from https://addons.mozilla.org/firefox/addon/foxtab/versions/

Any one have this problem?

Thanx dear this solve my problem.

I have another problem that in FF's previous versions than 4. When press f6 it select the address bar but now it select the tab

There is a bug report about it not working when tabs are on top. You can use the keyboard shortcut Control+L to go to the address bar.

An alternative is to use the F6 add-on to restore this functionality - https://addons.mozilla.org/firefox/addon/f6