I was online this morning and thought I was seeing an issue with my web connection. After some successful ping tests and also seeing IE working properly, I noticed a SSL dialog was displaying behind my Firefox window (version 7.0.1) and thus prevented any other pages to load.
I closed and restarted my browser to get back to proper browsing. I was also on Facebook, so it was very likely due to a certificate issue there.
Plugin wise, I run NoScript and Adblock Plus. Just wanted to share for anyone who may encounter a similar issue.