Images shrinking in FireFox

Hello –

I maintain several WordPress sites and have never had this issue. All my images, including the header banner, are appearing smaller than their dimensions in Firefox, and are therefore blurred. To make the header fit, I had to create it 40 pixels wider than the width set in my Thesis theme. I don’t know if this is Thesis, WordPress or Firefox, but the only new addition is WordPress 3.2. Any help would be fantastic!