I solved this issue with the following steps:

 1. determine if the problem is Firefox specific: I tried with Ubuntu Web Browser, and everything was fine, so it is Firefox specific (so there no dns problem for example)

 2. determine if the problem is Profile specific: I tried with a new profile (using `firefox -P` to create a test profile), and it worked, so it is profile specific

 3. then I found this question http://superuser.com/questions/467999/clear-301-redirect-cache-in-firefox I learned about HTTP 301 permanent redirect

I finally cleared the cache for the askubuntu.com url, which fixed the problem:

 - open the history panel in Firefox (Ctrl + H)
 - search for askubuntu.com
 - right-click on the entry, and choose 'forget this site'