Clear the cache and the cookies from sites that cause problems.

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

See also:

• Web sites or add-ons incorrectly report incompatible browser

I'm having the exact same issue, and the above did not work. I have also tried starting firefox in safe mode, disabling all plugins and using a new firefox profile. None of these work.

I hope this is not a hack, as I have the same Problem and I log on fine to paypal and bank via Explorer but not with firefox

Your PC is infected with a virus which hijacks your browser to display that message. It's a Trojan called ZeuS or ZBot.

It's only programmed to steal your username and password when you use IE, so they make that message show up in other browsers when you visit a site that it's programmed to steal your credentials for.

Tnanks for the info. I scanned my PC with a couple of scanners (ClamWin and Ad-Aware and they found and cleaned a few interesting items). Then Firefox 4 started working. Naturally I've changed all my sensitive passwords. Thanks for the tip.