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'

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 https://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'