A blank page for a new tab is the default in Firefox. You have an extension installed which changed that action, and now that extension is either disabled or it isn't working in Firefox 5.0. Take a look at your installed extensions and look to see what may be disabled - look for an update for that extension.

I'll try it. Thanks.

The horrible truth is connected to "google toolbar" they have not updated it for Firefox 5 and according to their forums don't intend to. This toolbar will work if you also install incompatabilty checker but the option to enable "new tab page" is missing. So far I haven't found a replacement -there are addons to use your home page as the new tab but not the recent history.

Please developers out there, have a look at thisand see what you can do - it was so useful