Well, well, well. See this : http://kb.mozillazine.org/Browser.cache.memory.enable where its mentioned that if browser.cache.disk_cache_ssl is set to true, secure pages will be stored in disk cache, not memory cache. Tried doing that and then fired off my site in Firefox and lo and behold, it worked! Of course, this was only possible by changing my browser config, so what I said in my previous post still stands.
Its interesting to note that wIthout changing any settings in Firefox, I went onto another site (of course this one was an ASP site without SSL) and logged in, viewed some secure info, logged out, cliked back button several times. It always returned me to the login page. On a more comparable scenario, I set browser.cache.disk_cache_ssl back to false in Firefox, went to forums.asp.net, logged in, Clicked on Edit Profile, clicked on Email tab, logged out, clicked back button. I was greeted with an Authorization Required dialog. On cancelling, it popped up again. On re-cancelling, it gave me a "401 - Unauthorized: Access is denied due to invalid credentials.You do not have permission to view this directory or page using the credentials that you supplied." page.
Sorry folks, but I have to say DNN needs to be spruced up!