I'm seeing it too. I think it's the result of a recent Hotmail update. It happens with Mozilla, Netscape 7.2, FireFox (all are based on the Gekko engine) as well as Apple's Safari browser (based on the KHTML engine). It's very intermittent, I had no problems at all yesterday, but today and last Friday were totally hit and miss. My guess is they updated something but only tested it on Internet Explorer.
Hopefully someone at Microsoft will fix it when they return to work on Monday or Tuesday.