Works fine for me on Firefox 3.6.8 - do you have that problem when running in the Firefox SafeMode?

http://support.mozilla.com/en-US/kb/Safe+Mode

Not related to your problem, but the Java Plug-in 1.6.0_03 for Netscape Navigator (DLL Helper) you have is too old for Firefox 3.6+ versions - you need to upgrade Java to get the Next Generation Plugin. http://www.java.com/en/download/index.jsp

"Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
"Remove the Cookies" from sites that cause problems: Tools > Options > Privacy > Cookies: "Show Cookies"

See also http://kb.mozillazine.org/Websites_look_wrong

Thank you for your attention, I upgraded my Java plugin and the problem remains the same in the safe mode also ( I already tried).

Thank you for your attention. I clean automatically at the win xp start both cache and cookies. The problem remains.

Resolved! Is a simply and handy way, but...

1) download the freeware Mozbackup from http://sourceforge.net/projects/mozbackup/ and install it; 2) launch Mozbackup and backup all your Firefox options; 3) uninstall completely your Firefox copy (by Revouninstaller, for example); 4) install a new Firefox copy; 5) restore the precedent backup except the option "general settings"; 6) resolved!

I don't know exactly wich setting created the problem, but now is all ok!