I just launched my website, which is fairly plug-in heavy, but I've also installed WP Super Cache. On Safari it loads quickly...even the Flash on the home page...and moves quickly from page to page. On Firefox, though, it's awful! There is a flash of background color before each page loads. It's VERY annoying as you move from page to page. I use Firefox for development, so I disabled all my add-ons, logged out of WordPress and tried it fresh but there was no improvement.
One thing I notice is that safari shows the <!-- Compression = gzip --> from WP Super Cache at the bottom of the source code, but Firefox does not. Since it is working and working fast in Safari, I don't think it's a hosting issue (shared hosting on hostgator), but I'm wondering why it would not be working in Firefox.