Another issue I've found is with the popular extension, Tabbrowser Extensions. I was having trouble trying with setting new tabs to open in the background, and I could not find information anywhere applicable to Firefox 0.9. So, curious as I am, I decided to resolve this matter myself.
Enter about:config as your URL, then scroll down to the text browser.tabs.extensions.loadInBackground
I've posted this information in a few other places to help those not knowledgeable about Firefox's configuration menu. Hope this helps!