Firefox's back button?

I recently updated my Firefox, then changed to Firefox 4. When i go to a webpage and click on a link that redirects me to another site, the back button will be disabled, disallowing me from going back to the previous site unless i can open it again. This never happened before the update, and I prefer it much more when it was that way. Anyone has an idea how to solve this bulls*** ( Sorry but I'm really p***** at firefox now )